The risk to user data is way too high.
This could have serious consequences. You could push client bugs with erroneous API calls, or server-side bugs that cause data loss. Rolling back isn't enough to fix the damage. The user's data has reached a permanent bad state that they didn't intentionally reach. You could roll back the data of every person who used the change, which would undo all of their work. You could analyze the data and try to fix it. This might work, or it might get the user data into a different bad state.
Plus, bugs in the view of the site might not cause errors that pop up in your error console, since it's hard to write tests for "looks wrong." Obvious errors - "when I click on my profile picture my name disappears" - are caught by external people instead of internal people, which adds a level of indirection between a problem appearing and a fix being written.
That being said, there are great uses for gradual rollouts. The video mentions that they do this for mature features with Gatekeeper - the developer can conditionally enable a Prod feature, and see what it does.
Unless you mean to suggest continuously integrating developer commits to trunk into the live branch, in which case, no, that's a horrible idea. Not every bug manifests itself that quickly.
As for merging, my memory is pretty hazy but I believe pushing and merging went hand in hand, and stuff that conflicted meant someone wasn't communicating yet working on the same code as someone else. Code was often documented with its owner. The code review utility at the time would (I think) take that✝, and (I think) run a blame and automatically CC those people on the code review, so it could be caught before it went out.
✝ Unless that was just done by convention so you know who to ask about a bit of code you might need to revise. Sorry my memory is unreliable on that bit.
That would mean having a lot of versions of Facebook live at any one point, but as those parts prove themselves stable they'd gradually be rolled out to all of their traffic.
One point that also wasn't covered is that as they're pushing things out they'll only cherry-pick parts of their codebase depending on which engineers they have around. I wonder if they have a lot of hairy merge conflicts around release time due to that, and bugs in production resulting purely from those merge conflicts. Or worse, subtle bugs resulting in change A going out, but being programmed against a function that was changed in change B, which is not going out because the author of change B isn't in today.