It's the Wrong Name
We’ve been calling this Trichodiadema decorum, mostly because it really really looks like that. See:
Until you see the blooms of course, and now that the plant at the top is starting to bloom, we very quickly realized that it is not Trichodiadema decorum, nor even a mesemb of any kind.
It’s Crassula mesembryanthemoides! I assume this means we were not the first ones to be fooled. I’ll post flower pictures after they’re open, any day now…