It doesn't have to be free everything to qualify for open source. The source and licence are available. You can actually bypass payment if you want to do that as the source ia there.

Right but it's a bit unusual. I was expecting the "pro" features to be in a separate repo or something.

I don't think I would even bother to update a local patch on every releases. I'll just use another foss app without silly "pro" features.

The code is indeed entirely AGPL, I looked at license headers in the specific files implementing paid features. I wonder if it’s an oversight. See my other comment https://news.ycombinator.com/item?id=42514304 for details.