He says ''so strong that I can't do it'', he just doesn't say ''its stronger than mine'', there is a difference here.
It looks like the difference must be on a completely different level to stop the teleportation, I doubt it's simply having stronger Haki would be enough to stop it.
I don´t really know if it need to have a completely different level or it at least enough to have slighty better.
I doubt Law could teleport or cut Katakuri or even Cracker, Vergo as example didn´t underestimate him.
Full body haki not mean>better haki, it already fail with Pica get easily cut by Law.
Also Doffy haki was cut by Law without haki shows enough that Law had the better haki, as example Law could block Doflamingo strings with CoA without getting cutted but Doffy couldn´t do the same.
Law power should and have a limit and Oda clearly shows here off, it was always straight about thinking DR Law could match Cracker, it make sense why Supernova without powerups wheren´t strong as a YC3lvl character.
As example I doubt Law could teleport Cracker who even had a better haki then Luffy.