I've heard this a lot on HN over the years but it doesn't make much sense to me. Some thoughts:

1. App tabs improves UX for 99.999% of users who aren't using a WM with a good tab solution (if one even exists).

2. WM tabs means launching a new app instance for every tab you might want vs having lightweight app tabs.

3. App tabs can do all sorts of app-level things and UX polish that dumb WM tabs can't do because they are so general. My terminal emulator tabs show a badge count of bell notifications, can be dragged around into groups, or dragging into other tabs as split panes. My browser tabs show you which tab is playing music and can impl right click -> mute.

4. I bet even the biggest WM tab cheerleader still uses browser tabs.

5. WM tabs are a different concern than app tabs, not a replacement. WM tabs are useful when you want tabs and the app doesn't provide a good tab metaphor or when you want to tile/group app instances a certain way. That doesn't mean it's not useful for the app instances themselves to have app tabs when it makes sense.