Here's the specific quote:
"When talking about older characters, Geno always gets a lot of requests. He didn't appear as a fighter, but I was able to achieve something kind of close to that as a Mii Costume."
This tells me two things specifically:
-Geno is popular among "older characters," not necessarily in general.
-The term "always" is used, which I interpret as Sakurai acknowledging the demand as early as the Brawl era.
I don't think Geno was a no show on the ballot, but I have my doubts he was especially popular. Probably enough to get the nod through the costume, like Krystal got an Assist Trophy, Shantae got the Spirit, etc.
There's also this:
"To tell the truth, it's because I wanted Geno to be a playable character. He has a gun for a hand, and I think he fits in really well with
Smash. During
Brawl, I thought it would be great if I could add him, but in the end it didn't become a reality."
So I think Sakurai has wanted to put Geno in Smash already since Brawl, so the costume is a reflection of that.
(Side note, why is SourceGaming's format messed up? All I get is a white background.)