I don't think they are clear they did not? In fact, they are clear that they did publish their key, because they mention depublishing it to test their hypothesi

I don't think they are clear they did not? In fact, they are clear that they did publish their key, because they mention depublishing it to test their hypothesis. The part they say they didn't opt into was that Proton (or any specific email client) would use the key by default.