That’s only true if you’re talking about the goals of open source/free software generally.
If we’re just talking about a game engine and releasing games, being able to modify the engine is absolutely critical when optimizing a large game. So having source available is absolutely a very practical thing when using proprietary software.
So it really depends on what you’re concerned about. Source available is just as good as open source in most cases if your goal is to build closed source software. If your goal is to build open source/free software, it’s awful.