Some of those things are true. I'm relatively sure fencing has been a combat style at points in the past; greatswords aren't used for combat these days either, after all, except in Ren Faire exhibition matches. If you're not using a gun, you're asking to be killed.
Considering Yi has exactly zero attacks that hit multiple targets via swinging his sword (Alpha Strike is essentially dashing through enemies, not swinging around them), two-handed doesn't make sense for "crowd control" for him either. Aesthetically, most things in the game are exaggerated - I think they actually made his sword larger with the rework, for some reason.
You're forgetting the most important point of all, however: if Yi wielded his sword in both hands, how could he easily draw the 5 other swords he's wearing if he needed them?