John Carmack on mutable variables

Is he referring to something specific with "true" iterative calculations vs. plain old iterative ones, assuming they are in some way "non-true" or less "true"? Like, avoiding i+=x in favor of ++i or something? Or maybe am I just tired today.

I think he's just saying that mutation is ok if it's something loopy, like changing the loop counter or updating some running sum. So both i+=1 and ++i are fine.