They’re less so forks than re-builds. They rely heavily on upstream and Google can certainly make it more difficult for them to maintain a “fork” that diverges too much from Chromium proper. Manifest V3 may be something they can easily keep out of their builds while maintaining the old method, but I’m no expert so I won’t comment on that.
A fork implies maintaining most or all of the code base themselves, when in reality, most Chromium derivatives simply change the “front-end” or the UI while keeping everything else upstream, except for a few tweaks here and there. To maintain an entire fork themselves would be a massive undertaking, especially since Google now has a practical stronghold on the development of the open web. “Forks” would have to ensure they work with the standards (and non-standards) that are being pushed by Chromium and there’s no easy way to do that besides using Chromium and “re-building” it with their broswer’s specific features and UI. Google could make Manifest V3 harder to exclude from those re-builds.
It’s this very reason that Microsoft moved to Chromium as they could not keep up with the changes Chromium were pushing to the open web. They have also made it very clear that they’re not “forking” Chromium for the new Edge, but rather using it as their “back-end” so they can focus on their “front-end” while contributing back upstream to improve the back-end. Too much of a divergence would land them back to square one with the issues they had keeping up with old Edge.
If a company the size of Microsoft are unable to maintain a “fork” of Chromium, it doesn’t leave much hope for the smaller companies like Brave, Vivaldi, Opera, etc. However, that does not mean that they are unable to exclude the Manifest V3 from their builds, just that they’re still beholden to the changes made to Chromium to some degree.