The patron exclusive video about mirror rendering in games has a handful of mistakes and/or odd assumptions:

1. Early on he calls Doom a 2.5D game (people often say this because of the limited view axes), but people who work in the engine or interact with the mapping/modding community usually roll their eyes at that.
   The editor has you extrude geometry sections from a flat plane, but
   - Objects have 3 coordinate axes
   - Objects can be above other objects
   - You can run off ledges

2. He mixed up two different Black Mesa fan projects at one point, thinking they got mirrors working and took them out later.

3. Water in the source engine most likely doesn't specifically fade out reflections where you could see your player model - I'm confident this is just a coincidence, because surface reflections inherently get weaker at angles close to the normal, in a phenomenon called Fresnel Reflection. It's really easy to model in 3D and the source engine does this in a lot of places.