My. God.
The compilers errors I was getting the past few days was for using the NODRAW texture to "paint" the back of my world brushes. While having the "portal2" keyword enabled.
Basically, when you have the "portal2" keyword and then you type "nodraw" in the filter, you get a NODRAW-like texture... which is actually called "tools/toolsnodraw_glass_footsteps" which is a translucent material, therefore not valid as a world seal for BSP.
Really...
If anyone has gone through the same **** as me I hope this post will be useful.
That explains the problems in my old map I was making, then I stopped because of this problem. :smith:
Welp, just another thing to remember.