He's right, these titles that Pirates have do not apply to Marines, and when Garp says WB was the King of the Seas this does not necessarily include Marines. Marines run the seas anyway, WB alone can't compete with them.
Marines are cops in the One Piece world, they're not supposed to be glorifying violence, but stopping it, nor are they competing with Pirates, but stopping them. No Marine will ever get a Strongest title, not even a strongest Marine title or even a reference to something related to strength.
Notice how Garp never got a Worlds Strongest Marine title? Heck forget title he was never even called the Strongest Marine or anything like that, at any point. That's because these things aren't something you are supposed to be glorified for as an enforcer. Imagine your neighbourhood cop being called the City's strongest and what kind of message that would send. Marines aren't out there to have dick measuring contests flexing their strength.
Garp was only ever called a Hero, something that Coby a fodder now shares with him. This is by design and is an appropriate honour for a Marine.
Having said all of that Garp is weaker than WB (imo), not because of WB's title alone, but because WB simply has better overall portrayal (which his title plays a big role in too) and a far more impressive toolkit. Infact I'd go as far to say that Prime Garp isn't even stronger than the Pre Skip Logia trio, because he doesn't have anything that cements him there.
