Yes, a grinder is another name for a submarine sandwich or sub. And I'm guessing the person who called it that is from Connecticut, western Massachusetts or Vermont, or possibly another part of New England. Here in the NYC area, we call it a hero. In Philadelphia and South Jersey, it's a hoagie. I'm sure there's a few other local names and I think sub is pretty universal in the US. I find it interesting to learn about regional differences and they often come up on the Dis.