No. I'm saying that it'd have to function on a fundamentally different level than with other CC to function properly, which is incredibly complicated on a balancing level, and probably just as much of a coding mess.
Knockbacks and pulls are duration+direction+speed. Direction wouldn't be changed regardless, so here it's duration and speed that matters. Your change would mean that duration would be changed and nothing else. If Riot just changed duration that'd mean Blitz would only pull someone 70% his normal distance if they had Merc treads, which would make them an absolutely massive counter against him because it'd cut out him ever using E on his target as well, since he'd never be able to walk up to someone fast enough.
To prevent that sheer scale of balancing nightmare, Riot would have to have it increase the speed at which the movement happens, which means the guy who got pulled now has less time to react to being pulled than before, which is another huge nerf to low ELO where people now don't even get to get past the "oh shit Blitz pulled me" state of mind before the pull is over.
And how would Yasuo's ult even function then, if he hit someone with and without tenacity at the same time? Like there's no good way to code it.
And blast cone - That's also a knockup, just not one Yasuo can ult off of. Dragon knockback, Baron knockback from entering his space, Knockups to move you out of player-created walls...You could end up stuck in all manner of things then.
There's so many potential issues that arise from making tenacity affect knockups.
So no, Riot probably couldn't do it. Not because they're incompetent, but because it'd be a fucking nightmare to handle it, and it just wouldn't be worth the manhours to do, since cutting .3 seconds off of a 1s knockup isn't really that important.