An engine is usually referred to as a compilation of various frameworks used as the core aspect of your game to interface with a system be it PC or console etc.
It's just a term really, I could write a graphics, sound, and input wrapper and release a game. I wouldn't call it am engine though. Engine references something a bit more robust and reusable.
A game runs a simulation, it has running logic, and it runs on an engine. You might choose not to call it an engine because for some reason you put unwarranted weight on the term, but that's still what it's called.
The point was ultimately that this idea that a priority engine is better for 'coding to the metal' seems redundant, because practically no one uses technology once at this point. You could say 'look at UC4, coded to the metal' or whatever, but the truth is that the engine was at the least started with Uncharted 1, practically a decade ago. It might stay in-house, but it's still not ground up, and thrown away after.
With tools running into the millions of lines of code, no one is building from scratch for a single AAA project, so while there might be some theoretical benefit, there are no real instances of it happening.
Moreover, if we look at the results of engines, is bespoke always preferable? Gears was the visual stand out for 360 for the majority of last-gen, and that was Unreal, the most ubiquitous engine of the generation.
