Knowledge of C (XROS was written in C and during the interviews the candidate rather uncommonly wasn't given a choice of programming language) and general understanding of how a computer works at a low level. Knowing the purpose of "volatile", understanding cache lines, mapping virtual memory to physical memory, DMA, this kind of thing.

I think everyone had a degree but looking at my degree (applied math) in particular nothing that I had learned at the uni was immediately useful and I think there isn't really anything that would prevent a smart person with a GED and some history of, say, Linux kernel contributions from succeeding on a team like this. Except may be a degree is needed for H1B visa for those who need it.