The point of the talk is it is non-trivial to detect those dependencies.

It looks like most of the time was spent discussing Python. I suspect that is because it is possible to create software without an explicit build stage, so you would not receive warnings about a dependency until the code is called. If the software treats it as an optional dependency, you may not receive any warnings. This sort of situation is by no means unique to interpreted languages. You can write a program in C, then load a library at run time. (I've never tried this sort of thing, so I don't know how the compiler handles unknown identifiers/symbols.) Heck, even the Linux kernel is expected to run "hidden packages" (i.e. the kernel has no means of tracking the origin of software you ask for it to run).

Yes, you can write software to detect when an inspected application loads external binaries. No, it is not trivial (especially if the software developer was trying to hide a dependency).

And just a quibble: even bootstrapping requires the use of a binary (unless you go to unbelievably extraordinary measures).