Sakazuki and Kuzan are not comparable to the current admirals. Just narratively Oda won’t give someone a 5 Billion Berry bounty just for them to be comparable to a 3 Billion bounty. That just makes no sense with how his characters discuss bounties.
It's like arguing Kidd and Law are = Mihawk/Luffy/Admirals because of a bounty. Whatever bounty Kuzan gets along the way, the admirals all scale of that.