I don't think it's really much of a surprise, a lot of those matchup rankings are looked on a bit rigidly I think. Dedede's a lot more than just the chaingrab although it's definitely a big ace in the whole. He's a great mix of stalling moves like f-tilt and swallow, quick and powerful moves, extremely high endurance, grab range, decent jabs, a multi-use projectile, flexible and effective aerials including what's probably one of the best back-air moves in the game imo, smashes that are good in their niche uses, and the aerial variety and maneuverability of Kirby and Meta Knight as well as an extremely good recovery move. Even if he didn't have the chaingrab he's still an extremely good character. He has all the tools needed to make a defensive heavy character work I think.
I think because Dedede is a character designed to be primarilly defensive, in a game that heavilly favors defensive play even for more offensive characters, that could be a reason too.