> it would just be another signal to fingerprint: "Oh, your browser is returning random values? Must be a Tor browser user".

That's perfectly fine! As long as they can't tell which tor user you are they can't track your browsing activity or associate it to any one tor user. That's the goal. Currently tor browser sticks out like a sore thumb by trying to appear identical no matter who uses it, which is fragile because any one data point unaccounted for unmasks everyone.