Residents of 12 states are eligible to participate if they meet certain criteria. But the agency’s plans have already met resistance from tax preparation companies.

It will reveal the username, not the identity of the author. If I tell you my Reddit username, what do you know about me?

My view is that Reddit has personally identifiable data but the data that is being licensed to Google, isn’t personally identifiable because the username by itself is insufficient to identify a person, without the additional data that Reddit isn’t passing over.
But I agree I may well be surprised by a DPA decision.