They would just statically account for it if they e.g., permanently modified the video

It doesn't sound like that's the case, per the link:

> This breaks sponsorblock since now all timestamps are offset by the ad times.

Sounds like either YouTube doesn't care about timestamp stability or they missed this in testing

SponsorBlock works by telling the video player to skip to certain timestamps within the native video player. YouTube however knows the exact offset of the ad that it sent, and this can offset its timestamps.

Could they update the plugin to do a location.href at the appropriate time?

No, because the YT frontend knows that a different video is played. It must know at least to make them unskippable. It's just that both ads and video are bundled inside one response, but the format is YT-specific.

I bet it won't. It isn't hard to offset, or better put : not take into account the ad in the timestamp (like the video plays the injected ad, but the timestamp doesn't run/increase/tick).