That's fair, you're right as I haven't seen statistics either. While I don't do it myself, I've seen many speak about doing this over the years though I've only lived in PA for a little over a decade.

Edit: autocorrect

He definitely could be Republican and I would not dismiss it at all. However, in the state of Pennsylvania it's common to register for the party primaries of the opposing party whose candidate is someone you're not in favor of and vote for someone else. It should be clear this does not mean he's not a Republican or imply that he might be a Democrat. It's only to add some context.

Source: https://ballotpedia.org/Primary_elections_in_Pennsylvania