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.
The microphone access is also for voice messages, which are supported by other messaging apps, but again, has to be listed in the health card - and looks nefarious.
