Also, consider what happens to someone new to a pool who is told to use bleach, but isn't told to add Cyanuric Acid (CYA) to the water. If they use bleach without CYA in the water, then it is very, very strong. It also gets used up very quickly so a lot gets added. And if it is added quickly in one place not over a return flow, then the denser bleach can pool at the bottom of the pool (for a while). If the bleach were poured into the skimmer then it could affect pump seals, again if there is no CYA in the water.

So there is some truth in what he says if bleach is used improperly. If you have CYA in the water, then the active chlorine level is much, much lower. If you pour the bleach slowly over a return flow, then it gets thoroughly mixed. For extra safety, you can lightly brush the side and bottom area where you add the bleach to ensure thorough mixing.

There are thousands upon thousands of vinyl pools using chlorinating liquid or bleach with absolutely none of the problems he was referring to, but these pools are using this type of chlorine properly.