If they are not entitled, then it's an empty promise. Only a contract can guarantee such revenue and nobody is going to spend their life time employed at this company.
There isn't really a great universal definition of a game engine, but most people think of it as a general code structure that can be reused, etc. It's quite common for new game programmers to want to make "an engine from scratch", but the simple answer that most people give is that you shouldn't. You should just learn/practice a programming language, then just focus on actually making a game. You can then take code from that game and reuse it to make your next games more quickly.