Yeah, they can't easily rotate keys, because the text can't tell you which key was used.
They could switch to a new key e.g. every month and then just check every previous key during detection. But that would slowly increase the likelihood of false positives, so no idea if that's a good idea either.

The section "4. What editing does to the mark" talks about that. Probably best to look at that illustration again, but basically those edits would interrupt consecutive runs of detectable text, but if a run is long enough, it can still be detected with statistical significance.
So, it doesn't have to check the 'color' of the words from start to end uninterrupted, but rather can also detect color sequences in the middle of the text.