You are right, it took quite some time. On the other hand, it looks like the legitimate part of contributing to xz was only a part time job for the attacker. The rest of the time, they either worked on the exploits, or in other things, like infiltrating other projects using a different handle.

Basically I can imagine the attackers being a well organized group, using work sharing and pipelining. Some members of the group would be preparing exploits, some would infiltrate projects and some would make sure not to get caught. And since infiltrating takes time, they would make sure to have multiple projects in the pipeline, seine in the early contributor stage, some in the social pressure stage, and some in the exploiting stage.