It's not quite the same your typical cycloid drive, but it has a strong relationship. Cycloid gears are about having an inner circle of radius A rotating within an outer circle of radius B by putting the center of A's circle on its own circle of radius C = B - A.
If C is small, then a fast rotation of C produces a slow off-center rotation of B relative to A. Since it's off-center, usually a bunch of holes (theoretically also of radius C, I think?) are drilled in B and filled with "output rollers" which connect to one single shaft which is on-center relative to C. Thus C connects on-center to this asymmetric B, which connects to an on-center output shaft that otherwise rotates as B does.
In his case, these holes don't exist and the rotation is never on-center. But you can use the same mechanism to convert his amateur cycloidal drive to one that has an on-center rotation of the input shaft.
Now normally in a cycloidal drive, A is regarded as fixed, while C is rotating, and B is still. In this case his C is totally implicit, his B is rotating, and he is at two different A's which rotate relative to each other. There are therefore presumably two different Cs, too.
It's hard to really isolate the exact mechanism in terms of connected cycloid drives. It is not as simple as fixing C and watching a single cycloidal drive's A rotate as a response to a rotating B, because that would have the reverse mechanical advantage. It's also not obviously "two cycloidal drives connected together by C", which would have a mechanical advantage near 1.
So I'm not sure what it is, but I don't think you can just say "oh, it's a harmonic drive."