He said he wouldn't hold back and then didn't, you're the one creating a headcanon that he was using 2% of his power or something.
It'll never happen, Oda's clearly going to worm his way out of including Saint Shanks in Mihawk's title, it's abundantly clear that Saint Shanks is his self-insert, he's not going to make Saint Shanks look bad just to make Mihawk look good, especially since we've already seen just how badly Oda can make Mihawk look in Marineford lol.
If Oda had wanted to weasel his way out of including Saint Shanks in Mihawk's title, he's done an absolute piss poor job of showing it
-He never had to make Saint Shanks be Mihawk's greatest rival
-Never had to directly mention Saint Shanks in Mihawk's bounty reveal
-Didn't have to show Saint Shanks only using his sword and haki to fight every time he does something
No one said he has to make Saint Shanks look bad in order to make Mihawk look good, that doesn't make any sense
And finally, Mihawk never looked bad in Marineford. You mfkers just can't get off the man's dick and will find any and every excuse to slander him. To this day he's never even gotten a scratch on his clothes and he supposedly looks bad lmfao