Oh yes they do. Have you ever tried to assign the .mp4 extension to open with something other than QuickTime? It just doesn't work, and it hasn't worked for a looooong time.
I've never seen any video open in anything other than my default video player IINA, it's almost like quicktime isn't installed, i've never seen it pop up.
When you change the file association on OSX, be sure to also click the “Apply for all .mp4 files” button. Common mistake.