This is a serious misconception of human cognitive abilities.
We have the ability to abstract generally - there is no abstraction for which we lack the capacity to comprehend. We regularly visualize, contextualize, and satisfactorily explain systems with dozens of dimensions. The fact that we cannot hold 4,5+ spatial dimensions in our imaginations sufficiently to develop an intuition for navigation in that space and geometry does not logically extend to human brains lacking the wiring or hardware for systems of thinking that are beyond our capacity.
We do have limitations in scope, in both memory and speed. Both of these can be overcome with augmentation and interfacing with UI or direct neural connections, and intuitive, comprehensive, deep understanding of systems can be learned.
You could very well know the underlying theory of how your 8086 processor works, how it interfaces with all the elements of the motherboard, how electricity and physics interact at each level of abstraction from transistors to the pixels representing the spreadsheet you're using to do your taxes. You won't be able to simulate that in your head to any significant degree of resolution.
We will require similar levels of system thinking to acquire intuition and deep understanding of complex new theories and models. AI can assist with that by providing UI for useful levels of abstraction and segmenting theories into chunks we're capable of consuming. BCI and augmentation will definitely allow a more total, holistic understanding, and I think it's the augmentation path that will keep us competitive with AI.
There's also a huge issue with your use of the word subjective - math is objective. Proofs remain stable whether it's humans or any other system that does the processing. We test that objectivity by comparing the subjective readings from individual humans, and if the tests all return the same results, we can confidently say that the resulting proof is an objective fact about reality. Subjective fundamentally means that depending on the subject, the reading might change. Modern systems of math are formally, provably objective. That's how and why things are the way they are; if they weren't, people would experience radically different individuated realities, or there would be clusters of results shared across some measurable characteristic of the universe. That's not the case, so you can confidently say that the foundations of our math and logic are sound.
You can even prove it for yourself - the abductive chain of logic that allows you to contrast your own consciousness and subjective experience, determine that it comes about because your brain is wired to "do" consciousness, like all the other humans, and compare your subjective reporting of phenomenal experience with all the other reporting of phenomenal experience, and achieve a ridiculously high level of certainty, in the Bayes sense, that you and other humans are conscious; from that footing, you can confidently navigate the rest of enlightenment rationality and formal logic and mathematics.
At any rate, Egan's mistake is one of kind, but of scale - I am certain that as we formalize and start creating any sort of universal proof library, we will find that useful and interesting things are of necessity a tiny fraction of all possible valid formulations of any framework of logic and math. Crude attempts, such as OpenCyc and other formal ontological reasoner systems, would need trillions of low level rules to have a rough approximation of the world model as complex as that of a human child. AI with trillions of parameters could probably start getting to the point where there's parity with human scale, but even if you turned the entire planet earth into computronium and turned it toward the task of understanding all the theory and science of the universe, there will always be far more left to explore and understand than the sum total of all knowledge.
All that to say, humans will be fine with ergonomic interfaces that map to human capabilities, even for extraordinarily complex and hyperdimensional systems.
> there is no abstraction for which we lack the capacity to comprehend.
How could this ever be tested/falsified?
It feels a bit like "there is no idea we cannot think of." If we can't comprehend it, then it won't be an abstraction, it'll just be a mystery.
> There's also a huge issue with your use of the word subjective - math is objective. Proofs remain stable whether it's humans or any other system that does the processing. We test that objectivity by comparing the subjective readings from individual humans, and if the tests all return the same results, we can confidently say that the resulting proof is an objective fact about reality. Subjective fundamentally means that depending on the subject, the reading might change. Modern systems of math are formally, provably objective.
You are of course free to believe in mathematical Platonism, but that doesn't mean that non-Platonists would agree that proofs amount to objective "facts" about reality. And you are equally free to "prove it for yourself", which will just end up begging the question unless you are a Platonist.
That's not to say that math is subjective. But claiming that math is producing objective facts ignores at least a few hundred years of philosophy of mathematics (if not more). Even practicing mathematicians like Chaitin have described math as being more about inventing than discovering.
Holding a non-Platonist position also doesn't immediately lead to the sort of constructivist / "anything goes" position that some people ascribe to it, where you'd suddenly lose the ability to say that 2 + 2 = 4 and not 5. There are lots of philosophical positions that would agree that 2 + 2 is 4 without also claiming that this makes it a "fact" about any sort of objective reality or Platonic discovery.