You guys don't understand that titles are the world/public opinion of the X era.
WB was seen by the world as the WSM and Said the WSC, these title are the public recognizing their strength.
The world/pubic of one piece have no idea that IMU exist, his existence don't contradict the title because they have no way to know that he is stronger.
Titles are not the end of it all the ultimate Truth of One Pieces narrative.
If IMU was public knowledge you would have a argument.
Dude, Imu-sama doesn't have public feats to support him being the WSM or even the WSC. We know that a title is given based on one's history in conflicts or the fulfillment of certain requirements, which Imu-sama doesn't possess. The funny thing is, when Oda said, "It's considered to be," people didn't take seriously the fact that what the fictional world thinks about a certain character is important for shaping their image. We learned this even in Ace's novel.
Your argument is like: Out of all the known public figures in the world, Whitebeard is the strongest. Out of all the known living beings in the world, Kaido is the strongest. However, does the mere existence of a daimyo from Wano, who is known only by a few figures in the country, make him superior to WB and Kaido? And that's the crux of the matter the simple fact that you are a little-known figure doesn't necessarily indicate that you are superior to others.
It's like he's trying to say that any piece of shit that works in the shadows out there and doesn't have his image publicly exposed is stronger than Wb and Kaidou, for god's sake.
I'm not really in the mood to discuss these conflicting absurdities that this manga gives us, but I lean towards the part that Imu needs a statement to support his superiority otherwise the fact that he is a hidden and little known figure is still not enough to invalidate the title of public figures who battled and made a name for themselves until they were called the strongest...