From the screenshot, his video is a mp4 so I don't think mkvtoolnix works for that, at least it didn't before, only for mkv.
Multiple people mentionned some so just pick one. i use mp4box with a separate gui but the new version complicates things so not the easiest to install.
Edit: Googling "demux mp4" will get you plenty of options, but here's a tutorial to do it with vlc which you likely already have:
https://bartman88.blogspot.com/2018/04/how-to-extract-or-demux-audio-or-video.html
Edit2: Oh, you can do it with mkvtoolnix, nice. Can't put anything inside an mp4 but you can take it out. Well, not quite take it out but put it inside an mka which is a mkv with only audio so should do the trick.