Really the point is that nothing can be injected into the git history undetected. To add any new code to the repository it needs to be put at the top of the 'stack' as the last commit. So even if someone got access to Linus' (or any other high level dev) machine they wouldn't be able to inject malware undetected as a look at recent commits would show the changes made.
Yep, the very detailed article comes down to one basic fact: "we always have lots of copies on our and other people's machines so we can always track ANY possible modifications."
In that regard, I guess it could be more "likely" to sneak in a very, very well hidden and cryptic exploit in contributed code.
Linus, however, is not distributed. If a malicious commit was discreetly slipped into his repository as a seemingly Linus-sourced change, there is enough trust in him that the change would likely propagate.
