While I'm not familiar with the tutorial you linked and I don't plan on spending hours watching it, I assume the swing speed of a sine based grapple would be based on the period, not the magnitude, which determines how far/wide the swing is.
Granted if you've been using a constant period, then a larger magnitude will be faster as it has to cover more distance in the same amount of time. Anyways, wouldn't it make more sense to adjust the period based on your starting velocity instead? Then you wouldn't have to fiddle with the cycle start position either, which I'm guessing works fine if you follow the tutorial.