So Primal Kyogre and Groudon do NOT get perma-weather as their abilities. I am genuinely surprised. Instead, what happens is that their special weather overwrites "lesser" weather conditions and cannot be overwritten by them (presumably they can still cancel each other out though). In addition to the normal effects of rain and sun, there are bonus effects which prevent Fire and Water moves respectively from doing anything - making Primal Groudon effectively immune to Water. Suddenly making it Ground/Fire is a stroke of genius, all that's missing at this point is Flare Blitz. Primal Kyogre doesn't care as much for Fire immunity, but hey, it was already vastly superior anyway, so no complaints. As for the duration, those conditions last for as long as the Primals are out, but fade away immediately upon switching out, preventing them from being as outrageously broken as they could be.
In my opinion, this is still an improvement over the regular abilities despite the potentially shorter duration, because with the weather nerf Kyogre and Groudon used their weather abilities for their own benefit much moreso than for passing it around anyway. And let's be honest, this is so much more interesting than just the return of perma-weather that it isn't even funny. Still, Kyogre didn't need the buff at all. Not even remotely close.