I think it’s pretty clear from the video that he’s removing material from the inside edge. This would also make sense due to the direction the trigger bar travels across the sear. Also, if you look closely you can see that the inside is angled, instead of square.
a new sear is like $10, you wouldn’t lose much if you f’d it up.
That said, should you really be modifying the internals of your firearm, if you can’t see these things for yourself?