I'm not sure there's a significant legal difference in the end. If someone could demonstrate that alternative browsers regularly get a lower score than Chrome, that seems like a pretty good antitrust case.
Or were you referring to the risk that individuals would sue Google for getting blocked from random, potentially essential websites?
Not GP but they most likely meant the second. The V2 prompt blocks people from accessing services, which could be construed as damages at scale.
You do bring up a good point about the V3 being potential antitrust issue, but that has always been a potential problem even with earlier versions of recaptcha. With V3, it's also deferring the liability to the webmaster. The action that the website takes with the score is up to them - in the end it's just a number.
Or were you referring to the risk that individuals would sue Google for getting blocked from random, potentially essential websites?