The guy has expressed an interest in writing an engine and there is a hell of a lot he can learn from writing that technology himself. He expressed no intention of making a game in his OP and so an insta-reply with a link to that article serves very little purpose and it is of practically no help.
I have never made a single worthwhile game in my personal time, i have always worked on various pieces of technology and still do today and that has served me extremely well in the games industry. Using Unity to make games however is only going to get you as far (in terms of your skillset) as writing games with Unity. I have worked on a commercial product with Unity and you don't learn shit about making games when using it. You just learn to use Unity and how to cobble stuff together.
If you really want to learn to make games you need to have a good understanding of how the stuff your game code depends on actually works. Game programming is not easy, its very technical. You have to be aware of all the same technical considerations as engine programmers do in order to do it well.
Since the OP asked about engines specifically (albeit it turns out to be a bit misguided in later posts) its important to just point him in the right direction with regards to developing that kind of tech.
However, in light of more recent posts in this thread it is clear that the OP is a bit misguided with his need to write an engine and if his intention is just to make a game then there is no better solution than Unity. I just object to that article being banded about as if its THE answer to every thread similar to this one
.