Is that available to other apps? If not - then this just looks like they're skirting around things they foist on third party developers to make themselves look squeaky clean.
I don't know the details, but I know that third-party apps can add iMessage apps. And the microphone input to replace/supplement the keyboard is a systemwide thing, working on any(?) text input.
It looks like the way it's implemented, access to photos comes through a Photos extension to iMessage (I think I recall these being called "iMessage apps"?), rather than it being inherent to the app.
And options for the microphone similarly seem to belong either to the extensions, or to the system—which handles microphone input as a substitute for typing.
It's possible you're right, and I'm missing something, but that's how it appears to me.