Once again, the first time Alex spoke, he did not know what number was on his hat. Since he is a perfect logician, what that means is that he did not have enough information to determine what number was on his hat.
However, the second time Alex spoke, he did have enough information to determine that his number was 50. It was not a guess, he knew it for certain, given that Bob and Chuck were also perfect logicians. So their lack of knowledge about their hats must have told Alex enough to figure out what was on his own head.
So consider your suggestion of 25 and 25: if Alex saw that, then he could reason as follows: either my number plus 25 is equal to 25, or 25 plus 25 is equal to my number. Since the former cannot be the case because that means my hat is 0, then my hat must be 50.
However, Alex would have been able to apply this reasoning immediately: if he had seen 25 and 25, he would never have said that he did not know his number. Therefore, 25 and 25 is incorrect.