That is a ridiculous comparison. Obviously, a Mewtwo with the ideal moveset for battling Chansey, and its higher attack stat, is going to be far superior agaist a single Chansey that doesn't hit back, compared with a Ho-Oh doing neutral damage.
Give the Mewtwo psycho cut / shadow ball, and make the defender a ZH/DG Blissey, and the result will be different. Or use them in a raid, where Ho-Oh's advantage in tankiness matters, and it won't be such a one-sided comparison.
Don't get me wrong, I still think Ho-Oh is underpowered and needs to be improved, but this video is an apples-to-oranges comparison.
As to your question, it is up to you whether to power up Ho-Oh or not. Personally, I am going to power up at least one of each legendary, to at least level 30, including even Suicune.