I see what they're saying with "don't guess" and I also agree with you. I'd phrase it something more like "explicitly acknowledge when you're guessing". It's fine to guess with the understanding that you might have to walk it back later if you weren't lucky. What's not fine is stacking guesses on top of guesses, and ending up marooned way out in maybe-land. "Maybe A, and then maybe B, and then maybe C... so it must be D." No, the chances of D for this reason are P(A) * P(B|A) * P(C|B)... ~= 0.

Also don't guess when it's easy to test. "Maybe divisorValue is zero" well slap a printf() in there and see if it is! Often you don't have to guess. I've seen discussions go round in circles for hours asking "what if it's X" when it'd take 2 minutes to find out if it's X or not.