To write good Fantasy (of SciFi), you have to go through a process called “World Building”

I think this is more implying that you don't have to work from the same framework for every fantasy world. Not everything has to be set in Arthurian Medieval Times with Crusader-Era social sensibilities. The menagerie of mythical creatures isn't a prerequisite or delimiter (dragons / unicorns / etc are not a requirement nor are robots / cthulhoid horrors / woolly mammoths disallowed). You need internal consistency (to a degree) but you aren't forced to adhere / omit any genre trope.

I would say, at an absolute bare minimum, you need some kind of fantastical or supernatural element to make it "Fantasy" as opposed to "Historical Fiction" or "Science Fiction" or some other category of fictional prose. Although, the genre of "Magical Realism" does make even that distinction a bit fuzzy.

many literature teachers / professors don’t even know about the idea of World Building

You don't necessary need to go through the whole work of World Building if you're just banging out a short story or novella. Even serial writers don't necessarily bother going deep on the background material until they feel the need to expand the scope of the setting. I mean, look at the Star Wars setting. George Lucas didn't have Jabba the Hutt defined as a big slug monster until the third movie. In the original film, there was a cut scene in which Han confronts Jabba, who was just a be-feathered chubby gangster.

If you're just spitballing or cranking out bits of fiction in brief, World Building can be superfluous. A story that takes place entirely in a single house over the course of a long weekend doesn't need the kind of scaffolding that a Long Walk to Mordor requires.