If you see an iOS only app, it's because Android users don't pay.
Sometimes you just need money to make things work, especially when one of your core goals is not needed external funding. Not everyone can get a 50 million dollar grant...
Also you should definitely elaborate on this because it sounds wrong:
> Apple offer APIs which make it easy for app developers to exclude and fragment
No idea what that's supposed to mean, and sounds like strange Apple bashing.
Closest thing I can think of is maybe iMessage apps? But that's a terrible example since the equivalent functionality on other messaging platforms also needs to be messenger specific, that's just kind of core to how they function?
You can easily copy my comment into Google Translate. An app available on the web, iOS and Android, free of charge.
I can't use this chat app (to my knowledge) to communicate with friends using this app without spending a lot of money to buy an iPhone or Mac.
It's also impractical for me to provide my comment in every popular language in the world. Its even more so impractical to write every comment in a multilingual way. There are two major phone platforms and it's not an onerous ongoing obligation to provide chat apps for each platform.