Honestly, I think its more like constructive criticism of KG. I don't think he's necessarily rippin' him to shreds. Basically, Simmons has consistently taken Duncan over KG over the years and is expressing some frustration here. He watches Duncan and sees that KG has that same exact potential but is lacking in the one department of low post back to the basket dominance compared to Duncan. This is a series where KG should absolutely be torching Atlanta down low. It's been expressed on the boards here but I'll reiterate, there is ABSOLUTELY no one who can stop KG down low and if they do manage to stop him, odds are it'll be a foul. What I think Simmons is saying is that he wants KG to be more assertive, BS recognizes the talent and its probably more out of remark of frustration in that one aspect of KG's game.
IMO, KG's unselfishness is a double-edged sword. His passing and ability to find his teammates off of double teams in nearly unparallel for someone his size. However, guys like Bird and Jordan also recognized the need to sometimes take over a game, even if it was the detriment of their teammates because they knew it was the best way they could win. (Before you jump on me for the whole he's a PF vs. SG/SF argument, I'm well aware of that, it's about exploitation) KG's unselfishness has sometimes prohibited it. There is an obvious weakness with an aspect of Atlanta's game so BS as well as others just want him to exploit.