That's really funny, because when I was a kid and first got a TG-16 when it was brand new, I called it a "Nintendo" also. My mom would get so angry with me because she would keep correcting me "No honey, that's a TURBOGRAFX" and I would persist with the "Nintendo" nonsense. After a year or something I came to my senses.
Either way, it wasn't because I didn't know the difference (I was an NES kid before I had a TurboGrafx), it was because back in those days "Nintendo" was pretty much synonymous with "video game console."