> It may be counterintuitive at first, but why wouldn't be possible?
For the same reason it wasn't possible at the time for a third-party to make N64 cartridges themselves to avoid Nintendo's fees. But I assume these lockout chips are replicated or circumvented by now.
Fairly recently the chip was studied in detail and replicated. But like most systems of that era, I believe it was circumvented fairly quickly by pirates. The easiest way would be to piggyback a licensed cartridge - unlike the NES, it has to be one with the same region and bootloader as there are several lockout chip variants, but that basic technique still works.
Related to this, every ROM needs a "boot loader" that has to be signed and is verified by the lockout chip.
Nintendo's boot code is under copyright and thus cannot be used. So the boot code distributed with Libdragon is signed by finding a hash collision with a brute force tool running on the GPU[1]. Fun stuff!
[1] https://github.com/DragonMinded/libdragon/tree/trunk/boot#bu...