While it is clearly a shitty move, it's not really clear to me that posts on Reddit consist of personally identifying information as protected by the GDPR.

Every post is tied to a username and email address, making it personal information, since each poster can be identified. I'm sure they're also tracking further metrics such as IP addresses, browser fingerprints, etc. It is immaterial if we from the outside are able to identify users, it only matters if it's possible given the data available to the processor. In this case, it is. Not to mention, there is a good chance texts and posts themselves contain plenty of personal information, such as linking to other user profiles, mentioning and discussing people, etc.

True, however I assume that Reddit is supplying Google with just the text. So, yes Reddit is collecting lots of PII, but that’s not what is going to Google to deduce it - unless you dox yourself in the text.

Not trying to be deliberately argumentative, just thinking this though, much as I dislike Reddit, the case feels weak

It doesn't matter, as long as the text is supplied as is, a simple Google search with the text and site:reddit.com will reveal the author, keeping it identifiable. True anonymization under GDPR almost does not exist, as it would destroy the dataset and make it unusable.

It wi reveal the username, not the identity of the author

It doesn't matter what it tells me. Personal data is clearly defined under GDPR as data that can be used to identify a person. It is irrelevant if you or I can do it with publicly available data, reddit has the data and that is enough to qualify it as such.

A DPA might absolutely disagree with my reading of the situation. I would be surprised, if a DPA considered usernames as non personal identifable information and know of no such ruling.