The title was never used to hype Garp, Sengoku also mentioned he had countless other legendary achievements.
Naturally, he's still portrayed there fighting the world's strongest crew though.
How does that mean anything? There were two reasons stated by Sengoku
-He teamed up with pirates
-By beating Rocks he ended up saving CDs and their slaves
Not Whitebeard either tho
Garp was stated to corner Roger in his own era not in Xebec's, so there's that too
Shiki is a mad dog, he's butthurt that Roger beat him so he's going around shitting on everyone lol
But yeah Roger's words "countless deathmatches" > whatever Shiki says in his rage fiasco.
And we know when Garp fought Roger, during his era.
Garp pulverizes mountains to dust with his fists, Shiki has a devil fruit that makes him extremely difficult to tag and has very high DC and AoE with it, don't know about Sengoku, but yeah naturally a fight between people of this caliber would half destroy an island, especially if with a fruit like Shiki's.
A 2v1 fight among the C3 for example would do much worse. Both marines weren't that concerned when he escaped, not that big of a deal
Blackbeard didn't think highly of anyone in Marineford including the admirals, he was high on Gura power afterall.
That still didn't stop him from fleeing with his crew upon hearing Akainu's name.
Marines act up to counter the great pirates, without pirates marines wouldn't be as strong because there's no one to stop really.
Whitebeard was the ruler of the seas, he was the most powerful and strongest pirate it's only natural, that title belongs to pirates nor marines. Whitebeard is the one who rules countless territories and boasts influence over other pirates not garp.
Roger's words during Marineford > Buggy's statement during an undeveloped time in the manga