yes, i dont see why the mixer cant just do that if he just want to remux an audio stream into a video container instead of passing it to another person. and if he actually wants to reencode something, the only reason i would know is if he wants to hardcode typesetting in there or something. like, what is exactly done besides just remuxing the mix into the video container
Because a mixer doesn't deal with the technicalities of video production?
There's a lot more that goes into processing a video than you think. It's not as simple as just adding the sound file to the video file and saving it into a video container.
Also I'm getting the feeling you have no idea what a mixer does. What they do is fix up the audio, add effects, and overall process the vocals so they sound good. The whole process it a lot more detailed than that but that's the overall gist. Their job isn't to combine all the files together despite what their title might seem like to you.