Yes, but read that again, he's making a new language, not a new engine.... To put it in terms of food, using things like Unity is equivalent to eating industrialized food, you have absolutely no control and you get what you get; Using other engines like Unreal or Godot that have open source is like cooking at home, some work but you can get it just the way you like; Building an engine yourself is like having a little farm in your backyard and doing everything from start to finish, it's slow, you'll face problems that have nothing to do with cooking that were handled by the farmers before and at the end you'll get something only slightly better than what you could using store bought products; Building a language from scratch is the personification of the saying "to make an apple pie from scratch first you have to invent the universe".

And you know the worst part? It won't be any faster or better in any mensurable way, large groups of developers spend decades to develop the languages we have today.