Because his DF's defensive abilities allow him to stay in a fight against an Admiral longer. The Admirals he faced were in the middle of a war for their honor essentially, had they lost their reputation along with the Marines would be tarnished so they were fighting much harder than Fuji was against Sabo when Fuji didn't even want to fight anymore he wanted to let the SH's win so he had stopped fighting hard.That's where I fail to understand. Why should we regard Marco as higher? He never done anything hurtful to an Admiral as well like Sabo...
No because of who it was against and the depicted impact is why we should assume it was a very strong kick. Sanji was leg checked by Vergo post skip that tells me his kicks probably aren't as strong as Marco's were yet. I'm assuming EOS he will have a kick much stronger than anything we saw from Marco but right now I'm not sure he does.
Just because the new Sanji has not clashed with an admiral or even met one we should assume that Kick was OP??? You seem to forget that Borsalino is a logia, 98% of times in the series Logia just simple watched/test you by tasting your attack. This was shown when Pekoms fodderised Caribou. He was tasting a Pekom punch which turned out to be >> what he expected