Update, hopefully this helps if anyone comes across this with the same problem. TestFlight was only showing me the iOS version of the app in the UI which is why it wasn't compatible. It was showing me the option as iOS app can be tested on a Mac with Silicon chip. What I needed to do is resend the invitation to myself and accept it from the email. It then showed me the Mac version and gave me the option to install it. Possibly a schoolboy error but I think TestFlight is "supposed" to show you apps you are invited to test regardless of an email. But maybe not, I could be wrong there.

Utter madness, I can't find one either