I don't think it's fair to compare anyone to Magellan pre-ts when haki wasn't a big deal yet.Ivankov got mid diffed, if not low diffed by Magellan. Zoro takes this.
Their wasn't an option of fight or flight against him, you could only run because all direct hits would leave you poisoned. Ivankov held off pretty much the longest anyone in ID would've been capable off thanks to his make up and other tricks.
if you look at other fights like Kuma, Sanji and Akainu it's obvious that ivankov was in different league pre-skip. The sh's couldn't hold a candle to him then.
I think based on that and possible growth he would still be pretty strong and probably stronger than Zoro still.
If Sabo gets credits to be 'admiral level' for being second in command of the RA, then I wonder why Kuma or Ivankov never got those. Or does that logic only work in one direction ? (read careful: I'm not saying that Sabo should be called Admiral level based on this, I just think it's weird that it happens)