So the vid wasn't of you with it fixed?
The starters have an overrunning clutch, like a car, so when the engine starts and revs., it won't spin the armature and throw the wire windings off, - and if the drive malfunctions/slips cuz of misc reasons, the starter will whirr, but often the small and large gear won't turn, or intermittently turn .
If that is the case, you just may need the drive on the shaft of the starter. ? ?