When I was making a Trident farm, I noticed that zombies were able to change to "Drowned" mob while just standing in the water, like it was that they could drown even in a rain puddle. I think that code just detects if zombie is in water block for long enough (about 36 sec), not checking if the mob has a state of drowning for a certain amount of time. Sorry for bad english, I'm Polish and it was a hard day for me so I don't think properly.