Like I said; "so what?"
Polling drops off within days. They'd already have a solid idea of who "won" the ballot almost as soon as it went up. You just don't maintain consistent traffic like that for six months. Nintendo weren't exactly signalboosting the thing every single day.
The fact that they pushed ZSS as a param update literally only implies that internally they'd come to the decision to make Bayonetta, but it does not imply that the character was already negotiated, especially since it took a whole 10 months until the character finally released. It's not as if her name is literally in there; the easy explanation is that they were hedging their bets, because despite the character likely being easy to negotiate it isn't a solid guarantee of her inclusion. If Bayonetta was fully decided before the ballot even started, I find it hard to believe that she'd release as late as February 2016, as the longer they take on including new content the less of a benefit it becomes financially - the game has less mindshare to go around, especially since it was also on a dying console (hell this is why other, more aggressively promoted games flaunt season passes because it's the most reliable way to make money off of DLC while the game is still fresh in people's minds).
Nintendo would have had nothing to gain by willingly bringing up the ballot and obfuscating the results. When Cloud was talked about in the presentation, they brought up that they'd received a lot of requests to include a Final Fantasy character - and bear in mind that the ballot even asks to specify what game/series the character stems from. It would've been easier to position him as the winner of the ballot, while Bayonetta could've been.... anything else. The fact that they decided on Bayonetta early is something I believe, but that doesn't mean that she wasn't decided on results that were actually real.