On the one hand, I totally agree that the champion creation process, as Riot currently does it, takes a significantly longer period of time than the estimates listed above (I believe the stated time frame is 6 months per champ).
On the other hand, I remember a time in league where a new champion was released almost every patch, and I am fairly sure that Riot was a smaller company then, with a presumably smaller champion creation group (though I'm sure they had more than one team creating champs).
Granted, some of those champs were notoriously broken upon release - but usually just numerically rather than technically (obvious exceptions exist, like Azir, though I do believe he came after the champ ramp was over with). It is also worth noting that champions were simpler then, with few of them adding new mechanics, effects, or technology to the game, so developing a kit was done in terms of what was already possible, not what would be cool to realize or expand on (Zoe has at least 2 new mechanics - sleep and walls adding length to skillshots. Not sure if re-casting pladdlestar counts)
So yes, Riot has shown that, in the past (with an arguably lower production value - fewer VO lines, very few new things, etc) a team can churn out a champion much faster than currently. I think it still took a team more than a few months to make a champ though.