The only HPX camera on the FCP X compatibility list is the 250. What's the most likely explanation? A real incompatibility, or just not tested yet.

I have no idea... But seeing as they both use an identical format there aren't any apparent reasons why it shouldn't work. They are both 100Mbit AVCintra or 50Mbit AVCintra, or DVCproHD.

370 clips load in FCPX and work fine with no glitches or anything, but yeah, AVC-I is AVC-I regardless of which camera it came from.

As to why it's not listed... I see stuff like this all the time, with some companies like Canon often removing products from it's website with no explanation, and not because they're discontinued either.

It may be that the 250 is the only camera they have had an opportunity to test with FCP X.

That said the 370 and any other AVC Intra camera should work fine as others have highlighted.