I don’t understand the actual mechanics of it, but my understanding is that it’s essentially like what happened with Volkswagon and their diesel emissions testing scheme where it had a way to know it was being emissions tested and so it adapted to that.
The malicious actor had a mechanism that exempted the malicious code when built from source, presumably because it would be more likely to be noticed when building/examining the source.
Edit: a bit of grammar. Also, this is my best understanding based on what I’ve read and videos I’ve watched, but a lot of it is over my head.