I agree with this.
The part about having space to lever out the waste is key as Ken says, and is also where Sellars' technique was non-optimal. In
the video he starts at one end and simultaneously works down and towards the other end, which is what I meant when I described it as "end-in" technique. IMO he develops an initial bottom profile that is overly "steep" (i.e. each sequence of cuts forms a very acute "V" shape) and lacks space for waste clearance.
Simultaneously working away from the center of the mortise in both directions works better for me, because the resulting "V" profile is much shallower. There are at least two other ways to achieve adequate waste clearance that I know of, and probably more, but that's the one that works for me.