Hmm,
Apple and cars?
I think folks fail to understand just how wishy-washy an "engine" can be. As I understand it, it's a big collection of tools and routines for handling the common things in a game. If you improve or even re-write some of those parts, it's still the same engine, it's just a different version of it. At what point, though, do you say it's a new engine? Kinda hard to say.
It can be the same with a computer. If you just swap out the graphics card, it isn't a new PC. Swapping the case sure makes it look different but the insides are all the same so that's still the same computer, right? Then put in an additional SSD. Oh, maybe it's time to upgrade the CPU/motherboard... At what point does it become a "new computer?" I think engines are much the same.