Depends on ya grading curve, definitely wasn't super quick twitchy explosive like Blake or whomever but he was ripped and a nba athlete. I'd reserve that descriptor for like Eddy Curry.
Apparently rusty & clams have a hell of a curve. Of course he was athletic because he was a professional basketball player, but when compared to his peers his athleticism was average. He was a big, tall, strong Ox of a man, but we're not talking about strength, are we? Like you said, he wasn't fast twitch,, he wasn't the least bit versatile either. He couldn't shoot, couldn't, dribble, but he might have been the best at the rim of his time. Here's a list, off the top of my head, of his contemporaries who were either a 4 or 5, who were more athletic than the mailman.
Barkley
Hakeem
Robinson
Shaq
Larry Nance
Shawn Kemp
Dennis Rodman
Larry Johnson
Alonzo Mourning
Derrick Coleman
Ced Ceballos
Tom Chambers
C Web
I won't even do him dirty and include players like KG, who played like 5 All Star games with Malone. There are other players I could include, but won't because the olds will cry, like my guy Vin Baker.
So, yeah, if we're composing him to Kirby Puckett or Claude Lemieux, yeah he's very athletic, but with his peers he's just an average athlete and that's okay, he's still one of the best power forwards of all time.