The crude variation of the attack-vector I'm describing is a backdoor, that is correct - but that's also what Thompson is imagining his attack being used for.

Thompson's scenario is more devious, more difficult to detect, and a lot, lot harder to implements (near impossible, as some of the SO answers point out). But at that time, people didn't generally download millions of lines of code written by perfect strangers and incorporate them into their programs, so that was the only way to pull such a thing off.