Rule of thumb (and the other fingers I suppose): if you are getting sore quickly (or really much at all in most cases), you are doing something wrong. Piano is more about finesse and technique than physical strength (or else professional pianists would have massive forearms!) Sure, there is some strength and endurance to be built up over time, but this comes as a natural side effect of good technique. Pushing through soreness in hopes of "building strength" has the potential to actually do more harm than good.
When it comes to playing tremolos, like the ones you mentioned, the most common tendency is to find the hand shape for the chord, lock it in place and rotate the whole forearm at the elbow. This takes a lot of physical exertion and will make you sore very quickly. On the other hand (no pun intended), playing with your fingers only can be just as bad.
Rousseau is an excellent pianist, I am a big fan of his as well. Notice, in the video, how loose his fingers are, particularly the ones not actually playing a note. Also notice how the back of his hand is rotating slightly back and forth. The key here is to use both your fingers and your wrist/forearm, distributing the exertion between them. It may not be exactly 50/50, you'll have to figure out what proportions are most comfortable to you. Once you find the sweet spot it should not cause you any strain.
(One other neat trick to conserve just a little extra energy when playing the same note repeatedly is to not lift the key all the way up in between notes. Once the string has been struck the hammer falls back to its resting position and can be re-engaged by lifting the key only part way before depressing. This means your finger doesn't have to travel as far, reducing the speed at which you have to move it.)
