Those polls is what leads to believe Isaac still has a chance to appear as a DLC, right?
That's just part of it, a small part of it. That's just an idea of what was the actual demand around the ballot times, that dojo poll specifically, since it was like the biggest one back then. He was always at the highest spots in every poll including that dojo one which is most likely the most accurate we could probably have around the ballot time. ( In fact if you check it, we got almost everything that was around the top 10 places or so, which was banjo, veterans, wolf, snake, etc).
There's also the situation that both GS and Arms shared during launch of Ultimate, which was lack of music tracks on Arms' end and having 2 tracks that don't even have anything to do with Isaac (the boss from the second game and the overworld theme of the 3rd game), literally not even a new song from last time we had his AT, nor his actual fighting theme. Also spirits that should've been all part of those series were missing, while just having a few of them. Both being shown in that very last presentation before launch while having those weirdly lacky compensations (yes, despite still getting miis, AT, and spirits, it was super weirdly handled overall in comparison to every other AT and series). Arms fixed the music and spirits issue and Pushdustin translated stuff from Sakurai during Min Min's release:
“In order to add content from upcoming games to Smash, Sakurai was able to see a number of Nintendo titles that were still in development. Since it was so difficult to add a character, Sakurai decided to add Spring Man as an assist figure, and give ARMS some spirits.“
Which heavily implies that Arms was meant to be in base roster, but due to "most likely" time limitation and an Arms fighter being more complex to develop, they decided to just go with a mii and an AT and a few spirits at the time. The only reason Min Min didn't happen in Pass 1 is because Pass 1 was also already decided at the time, so Arms had to wait aaaall that time until more chances of DLC to happen, which is what Pass 2 is (in fact pass 2 is described as "more opportunities to satisfy fans"). Isaac's music and spirit situation has yet to make any sense and be fixed, and no other AT aside from Arms and Isaac had this weird way of handling its content for its inclusion in the game.
Now we have ALSO Bomberman's AT happening now, which pretty much proves that they are listening, even now for Pass 2, to base roster backlashes. Since Isaac had a HUGE backlash back then (which even my source confirmed to me back then since he was in contact with people in the company of course), and he has already an AT and a mii fighter, then the next best thing to make up for the backlash is just a fighter.
It's just a bunch of small things here and there that when piled up it really makes you see how likely he actually is in the current look of the situation. And well, if there's that new game coming whenever the hell Camelot decides to announce it… then it wouldn't be surprising if they kill 2 birds with 1 stone by having Isaac not just satisfy fans but also help marketing the return of the series (altho this doesn't NEED to happen for him to be likely anyway, it's just an extra possible plus for us lol, probably a wet dream at this point by how it sounds).