You are correct here, managed code is indeed absent in compression and encoding. However, the story is a little more complex. Code like:
In areas where performance is very important managed code is indeed absent (compression, encoding, rendering, etc) due to it's performance penalities.
Pretty much. However, it could be that a specific combination of (invalid) OpenGL states only appears at a specific location in the game. Or it could be that that area contains an asset with a problematic shader. It happens, and my point was that the more you trust your compiler the better.
Well I can't say I've done much game programming (and even less opengl programming), in fact it was many years since I did anything remotely as such, that said I can't imagine a game where you'd write special opengl code for a secret area. As I see it you write an engine which handles all drawing, and that engine is thoroughly tested during development, a secret area just like any other part of the game is defined using tools (map editor, event editor, etc) which in turn access the drawing engine using a higher level interface and not by anyone hacking together specific opengl code for that part of the game.