Considering the theme appears to be "Make the new UI look like XIV" (probably to make maintenance/add-ons easier), it's likely that it's not that a gamepad flat-out won't work in the new UI, just that it won't be as optimized for it as the old one is.

There are a number of things I can do with a controller in XI that I cannot in XIV, but XIV is still playable with a controller overall.