You are are confusing dangerous and socially dismissed with stupid.
Back when people didnt know that horses were safe to use than yes it was probably seen as crazy and a waste of time/resources to ride a feral beast into battle on its back as its trying to throw you off and is afraid of pointies and sound.
but chariots, as he pointed out, were the starting period. We bred stronker, larger horses to carry us faster and with more stuff on back. most likely people imagined an actually personell carriers, taking whole groups of soldiers or archer to a flank position and unloading them.
With stronger whorses, who didnt fear battle (at least from a distance, biut plenty of them were shock troopers), they were halfway to riding them.
The could be handled safely, could bear a weight on their back (pack animals), and were conditioned to battle. It was, as again he said, simply a matter of developing a means to let them climb up and control them while holding weapons.
He has a clickbait title to the video, but he more or less lays out the same thing I have just said in his video. I should make my own videos nao.
What his video title tells me is: "Layman! You know how you call something stupid because you dont think it can be done? All great innovation is people doing what everyone thought could not be done! Look at the horse for example; it is so common you take the process of domestication and development of saddles for granted! Bow before my ranting! Down on your knees Im the
piano god!"