You can style your whole webpage with divs, but using main, nav, footer or whatever blocks is semantically more correct, because you group elements together that have a certain purpose.
A HTML Tag in the middle of a sentence is not wrong per se, but when parsing it a line break could signify two sentences where one has missing punctuation, instead of a complete sentence as your original intention was.
I don’t really care how the design you want is achieved to be honest, but I don’t get why the prof didn’t argue against.