FLoC is not immune to this: it relies on the device being able to track users and then provide advertisers "blurry" access to that tracked data. The problem is that we already have plenty of other tracking mechanisms that we can't reasonably restrict and that will interfere with the privacy protections built into FLoC. You will always be able to fingerprint and FLoC will always provide some fingerprintable entropy.
Even if FLoC was trustworthy enough to do what it claims, your interest cohorts alone can reveal your secrets. There's the classic example of Target knowing a woman was pregnant before her father was, for example. Yes, Google is going to try and filter out sensitive interests in cohorts, but that's an additional layer of trust you can't control. What if Google's definition of sensitive interest differs from yours?
