Your rooter guy is not a plumber. He snakes drains perhaps, but it doesn't make him knowledgeable as a plumber.
All toilets since 1992 use 1.6 gallons or less.
That is the conventional knowledge.
Tanks haven't fully drained now for 19 years.
If you have a comb in your toilet, using more water wouldn't have helped. Combs are for fixing your hair. Keep the comb out of the bowl, and you should be fine.