the problem is I already had TestFlight installed, because I was an ARC tester/user. opening TestFlight only gives an option to run ARC.
I solved this by finding the URL in step two in desktop safari, manually entering it in mobile safari, then clicking the “start testing link”, which took me to the App Store to download the test version of roon remote.
I installed it, overwriting the existing version, and all seems well.
the messaging about this is very poor, and IME, very un-roonlike.