Here it comes the haki cope, Luffy used Haki against Kizaru, no average reader is going to read Kizaru dogwalking G4 Luffy and think:
"No, that Luffy is Doflamingo level because there wasn't black ink on the paper".
Kizaru is undoubtedly above every iteration of G4 Luffy and that's what Oda wanted to portray, hence Luffy going G5.