I call BS if they "take points off" for not knowing the exact term "joint table", and if so that's not the kind of company I'd want to work for.
Sadly, looking for excuses to rationalised that the interviewer doesn't like you is still a thing.
Edit: On the other hand, asking the question in a way that gets at the concept, not the term, seems very reasonable.
I don't think it's relevant, but yes, I have interviewed at such companies.
I don't believe OP is aware of the existence of table joins, and just reproduced what he thought was mentioned.
Meanwhile, the DB operation he described is clearly a table join (create a new table by creating a relationship between two existing tables).
That is also quite clearly a good reason for a prospective employer to lose interest in OP as a potential recruiter, and why his projects on Github fail to impress anyone. Portfolio projects only work in favor of a job applicant if they are any good, but if they are all a bunch of crap that is poorly put together and showcases incompetence, such as setting up a database system that tries to reinvent the wheel to fill in the gap of not knowing the very basics, then obviously that is disheartening to any potential recruiter.