Imo he is still on par with Law powerwise, then Law has a good mindset while Kidd is always in a Leroy Jenkins state, and that makes a difference in the end, anyway 4th supernova all considered, behind Luffy, Zoro and Law. Also even if Law was fighting Shanks instead of Kidd I think that Law would have done only a bit better, he would have resisted some attacks, maybe, by teleporting a bit around but in the end Shanks one shot or 2 shot him too. Afterall Shanks at this point should be still stronger, and quite a bit, than BB, imo that was something Oda wanted to show with that double clash of Shanks vs Kidd and BB vs Law. Also, even if it is not canon, Shanks in film Red had his sword at Kizaru throat in a couple of moves, and Kizaru was effectively speedblitzed at the end of the fight, so if Shanks can really speedblitz Kizaru then people like Kidd or Law are doomed from the start so Kidd character for sure was destroyed but that clash was mainly, imo, to show Shanks power.