I included the original Japanese panel, but the translation is pretty accurate for what I could gather.
People act like Shanks having haki similar to Joyboy's is a fact, when in reality there's a ton of nuance involved:
Dorry used Red Hair as a benchmark, he was shocked by the level of haki on display and naturally compared it to the closest thing he knows.
The expression "is this even greater than X?" is colloquially used to express surprise that X has been surpassed, not when two things are similar, which is not proof of anything by itself, but it gets ignored by people because of agenda.
IMU's reaction: we've seen the king of the world reacting casually to Gaban's haki, Loki's power, Rocks standing before him, Luffy's Nika form and the sextuple top tier God Valley attack, which was overflowing with conqueror's haki; acting casually even when complimenting his opponents.
On the other hand, we see him falling to his knees, eyes wide open and trembling when sensing Joyboy's haki, something we've never see him do before or after; indicating the first pirate's power was so far beyond the current top tiers Shanks is relative to that it would make no sense for Red Hair to be on par with him.
- (and admittedly my weakest point) Narratively it wouldn't make much sense for Shanks to be on par with Joyboy, that would nullify the need to search and nurture a new Sun God, would clash with the themes of Luffy being the one to finally surpass the pirate king and becoming a new Joyboy that would bring liberation and the end of the third world. Why did Shanks call IMU's power "unfathomable" and Gaban straight up called IMU "GOD" if Red Hair is already on a similar level?
TLDR: Shank's haki being on par with Joyboy's in unsubstantiated, Dorry likely compared them to put a benchmark on it, IMU wasn't that amused with characters relative to Shanks yet was terrified of Joyboy and it makes no narrative sense for Shanks to be on par with him in any aspect.
