open in app rule

That is not a deliberate feature. The app is simply not registered correctly for the type of link. This registration happens at the OS level and can propably be set up after installing the app ,depending on your OS.

Conversely, if I don't have their app installed, don't assume I don't have another I would like to use. Re-directing to the app-store (especially the Play store, Galaxy store, or whatever Amazon calls their non-sense) is never the right answer, at least for me, nor do I want my kids or anyone I provide tech-support to downloading apps like so.

if this happens to you in ios consistently for some broken apps you can work around it by long pressing the link and manually choosing 'open in app'. The reddit app used to do this to me when i used it and i never figured out why before i deleted it but the workaround always worked 🤷🏼‍♀️

I have genuinely not seen that. I always get taken to the app store when I don't have the app installed already. I thought this is an os-level feature...

Absolutely