At the end of the day, even if you assume good faith on Google's part (which I think is quite a leap), causing the user to present more entropy to the site will make them easier to fingerprint.
256 topics would be ceil(log2(256)) = 8 bits of entropy
30,000 topics would be ceil(log2(30000) = 15 bits of entropy
As a reminder, there are ~ 10 billion people on earth, so if you have 34 bits of entropy or so, you can uniquely identify each person.
So really, the way to think of this as "Google considers making FLoC 20% less effective at fingerprinting users", and that's not even considering other sources of entropy, like user agent or screen size.
256 topics would be ceil(log2(256)) = 8 bits of entropy
30,000 topics would be ceil(log2(30000) = 15 bits of entropy
As a reminder, there are ~ 10 billion people on earth, so if you have 34 bits of entropy or so, you can uniquely identify each person.
So really, the way to think of this as "Google considers making FLoC 20% less effective at fingerprinting users", and that's not even considering other sources of entropy, like user agent or screen size.