According to Paul Allen's book [1] about his time at Microsoft (admittedly a biased source), his particularly critical contribution was an 8088 emulator/simulator for the PDP-10. That allowed them to write and even interactively debug (if I remember right) BASIC for the 8088 on the PDP. It would've been hopeless to develop directly on a microcomputer, so they would've had to have written on the minicomputer, transferred the binary across and see if it worked, and iterate like that.

That contribution wouldn't show up directly in the BASIC source code, since the emulator wasn't part of BASIC itself.

[1] https://www.amazon.co.uk/Idea-Man-Memoir-Co-founder-Microsof...