It's really ambiguous. I've been saying since January that Hero was probably the first DLC character decided, likely to have hit snags due to legal issues (maybe even originally been planned for base) and I seriously doubt it had an impact on Geno's chances. Beyond that, there's a lot of things that we've talked about in circles:
-Sakurai's statements in 2016
-how the Geno costume was treated when it came out
-Geno not being an assist trophy despite how other similarly popular characters ended up this way
-The above point made even stranger by how they bothered to negotiate AT content for Monster Hunter, Fatal Frame, Shovel Knight, and even Virtua Fighter
-Nintendo's odd nods and references to SMRPG
-The Vifam figure that we can presume was used for the movements and poses of a future character
-Banjo ****in' Kazooie being in the game, torpedoing a lot of the cynical views that people had about Nintendo being involved in choosing characters
-How Geno not being in the game at all at launch would have been a huge red flag
-How the costume isn't necessarily a death knoll since it would be a simple port instead of being totally brand new
-How Joker, Hero, and Banjo pretty much prove that Sakurai can add whoever he wants and that Nintendo's probably happy to help make it a reality
Honestly, I don't know. My thoughts right now is that, if he's in, he's probably the last character because he fills a very similar niche that Bayonetta did in Smash 4: kind of an obscure character that has a lot of significance with the Smash fanbase. As things right now, I can't think of very many characters with his level of popularity that fill this niche.
Consider this data. 45,000+ votes and Geno's in top ten for newcomers and top 5 for third party newcomers. It's very HMMMM inducing when you look at this as a whole and see what became what, including a couple of characters who did notably worse than Geno become playable. Going into E3, I was more on the negative side but Hero and especially Banjo were definitely game changers so it upends a lot of thoughts that I previously had.