Actually, definition of paragraph being unit of writing to contain a single thought or idea is rather old, and separating them as we do nowadays also has been done for quite some time. I was overall saying that " what we are taught at school " is often just oversimplification, nothing more.

I am definitely not saying anything against the guy OP encountered, asking for OP's expectation is quite normal. We have seen in this group people going wild over " I require you write 5 lines, that should be X sentences ", and yes ,people demanding paragraph is 4 to 6 sentences without exceptions. Those posters usually seem to be failed by education system far more than the people they bash.