Right you are. I like to think that I'm fairly knowledgeable in this space, but Threema totally passed me by. Maybe it's more popular in german speaking communities (or I'm just more than I thought)?
(this is a US-centric comment, as phone numbers are rather static here; unlike other countries, not having a number isn't really how it works for most people on the big 4 carriers and their secondary MVNOs)
48hour refunds on google play should have addressed this conundrum but it seems people just aren't aware of it :(
But there's not a single non-German contact on there.