That's how our larger on-prem customers use the iOS client when they run their own in-house control plane server.
A simple setting in the app would be far easier yeah. I would suspect that the ease of people bypassing the paid service is probably not a priority for them ;)
Personally I never even tried tailscale as I try to avoid Google. I definitely don't want to use my Google account to log into it and give Google more information.
But tinc serves my usecase well. The peer injection is a bit of a worry there too though. Especially because tinc peers are able to add any peers on their own (it's a feature meant to provide easier configuration).
I tried nebula extensively too but it didn't add enough over tinc to make it worthwhile switching especially now that tinc has an Android app.