That is a wholly intentional boosting of the skill using glitches and exploits.
That is absolutely not possible with 100 athletics, again which takes 95 hours of continuous running to achieve, not even if boosted. I suspect that video has to do with the glitch/ bug that allows you to bypass the potion cooldown timer, letting you drink as many as you want.
And even still, it is a better game that includes at least the option than a game with a lot of hard caps, boxing in the gaming experience even more.
Yes, of course.
I did level a character in Oblivion with such skills years ago and still felt it was a bit much though.
The problem I had with it was that it was too easily sustainable. If I were to change it up with Skyrim, I would have made fast running an additional set of perks that trigger with holding sprint for a few seconds, that further enhances speed like a short burst that moves you at double the normal sprinting speed.
With jumping perks, I would have had large leaps possible by holding sprint while stationary for a few moments, then either jumping straight up or making a running leap. Their greater benefits should not be largely passive like in the past, and instead should be active and consume stamina.
And again, I disagree with your last statement. I know people dislike limits, but they are sometimes needed. Maybe not hard caps of course, bit diminishing returns instead.