Anything divided by 0 is infinity, theoretically, but practically speaking it is undefined. Either way, it is not 1. mIRC returns 0 because a computer can't represent infinity except via symbol, and it's not appropriate for mIRC to return a symbolic value. As stated, you should do your own sanity checking on valid calculations.
I know it's a minor point, but the part in red is not actually correct. Division by zero is undefined (both theoretically and practically speaking). Infinity is a concept, not a number, and therefore cannot be the result of a mathematical operation on the real numbers.
Anyway, that doesn't take away from the point of your post, which is that Khaled decided to give nonsensical results for division-by-zero calculations instead of having to deal with throwing/handling an error.