And more relevantly to GitHub, they still use Azure and VS Code and Surface computers and XBoxes and keyboards and etc. etc. etc.
I don't expect Microsoft, as a company, "to go around snooping competitors' source code any more than they are going to go around snooping competitors' email" or Azure infrastructure or code editor or putting loggers in peripherals or any other tinfoil theories.
I _do_ expect some companies to reconsider their policies in light of the acquisition and decide that other options make more sense for them for numerous reasons, including but not limited to not wanting to hand (even more) money over to a competitor for a service they don't necessarily need Microsoft to provide to them, or if they suspect Microsoft will significantly change the existing ToU/ToS in ways those companies would rather not deal with.
Microsoft isn't going to go around snooping competitors' source code any more than they are going to go around snooping competitors' email.