Honestly with how merciless and swift the de-confirmation came on Waluigi, Takamaru, Ashley, Bobmerman, Krystal, Zero, Knuckles, Shovel Knight, and Sukapon came, all of which were characters with mid tier, to upper mid tier, or even lower high tier amounts of fan support, I have a hard time believing Isaac wouldn't have been among them already. Isaac is obviously high profile enough to AT LEAST be an assist, and with that in mind, would it have not been wise to be just as swift and merciless? He's an assist that would funnel speculation a lot. People would focus moreso on geno, or potential modern characters like Rex or Springman. But the fact that he hasn't assuming he isn't an assist, has only served to divide the fans in terms of speculations and desire.
But I believe that Sakurai would know that already. He would have the foresight to know that holding onto a high profile assist for that long would only make the damage of thier deconfirmation hit harder then anyone else, that's probably why they debuted Assist's WITH Waluigi, arguably the most supported assists to be promoted going into Ultimate from 4. And don't tell me they wouldn't put him in the game at all. They have to acknowledge his popularity made evident by the ballot.
It's for that reason I believe Isaac was an edge case. An extreme edge case. Any fewer votes then he probably recieved, and he wouldn't have made it. But he got just enough to push him past fellow ultimate assist's, Bomberman, Krystal, etc. Into a playable character. No other explanation makes sense to me. He was certainly more popular in the ballot then Bomberman and Krystal, so where is his assist? The answer is that he's playable. Otherwise he'd have had his assist debut in the E3 demo, or the august direct at the latest.