It's not his height that makes him a non-rim protector. It's his lack of height, lack of length, and inability to jump. Alan Williams looks like a good rebounder and maybe a good position defender in the post as he's got a load in his seat. But he will never be a rim protector and his offense is EXTREMELY limited because he lacks the footspeed to take anyone off the dribble, doesn't posses an outside shot, and is so earthbound that he can't score inside effectively. He has shown ability to get to the charity strike, but that's in summer league. In regular season I think his shot will get sent back consistently.
This is not to rip Alan Williams. Rather it's a pretty unbiased view of who he is and who he isn't.