Most apps on the list are lemmy apps, meaning they don't work with kbin. Artemis is specifically designed to work with kbin, not sure if or when any of the other ones will go in that direction or become interoperable as there are some challenges with the kbin API at the moment. Having said that, a new API is in the works (https://codeberg.org/Kbin/kbin-core/pulls/357) so things should get better with time. Some of the apps are in very early stage of development so it may happen that they adjust OS availability and platform support.
Mlem is no longer on testflight, they just announced a change in leadership, so hopefully it continues with development. Seems like it will, given their announcement: https://beehaw.org/post/693759
That’s strange. I just received an update overnight for Mlem on TestFlight. Maybe it’s just closed to new registrations? IMO it is the most polished of the iOS apps.