Yes, but there are quite a few other subscription services which don't offer an IAP as well. This just feels strange that they would suddenly single out this app.

They don't single out any app. They do inconsistent reviews, humans are doing it, and sometimes they'll flag you - and you're more likely to be flagged if you aren't a multi-billion dollar app they are familiar with.

The Apple note quoted in MacRumors article mentions a class of app that is a "reader", aka, subscribing to content.

Netflix and Spotify, as well as Kindle, various news sources, etc., their apps are "readers" aka consume only.

Perhaps Hey is not a reader?