I do think this is closer than most people think. If DM were our primary ball handler, and had the same weapons Simmons has around him, his assist totals would go up commensurately. But given how depleted our team really has been, and the fact that Simmons had other strong players to lean on while DM was given the keys and told to make something happen...which he did...it is closer than the stats say. Mitchell has not been expected to be the same player Simmons has, he was asked to shoulder the scoring load, and play solid D on the other end of the floor.
In terms of assists, our team is far different in this regard than the 6'ers. We have no real single play-maker, even Rubio's assists are WAY off his career averages, yet our team shares the ball well. It is a team effort, and with players like Ingles, Rubio, Mitchell and even O-Neal, that can all contribute in this area I would never expect his assists to be that high. If he were dominating the ball that much, we would likely be a worse team. Simmons is pretty much the sole play-maker. I would expect his assists, with the weapons at his disposal, to be much higher. Remember, every Stockton needs a Malone and a few other people to get that particular stat. If Simmons were playing along-side a Rubio and even Ingles I would expect his assists to drop. If Mitchell did not have them, I would expect his to go up.
Rebounds I concede, but there is an argument here as well. Mitchell could make some improvement here, but given his role again, I do not expect him to get many rebounds with a Gobert and a Favors on the floor. And he does get some improbably rebounds from time to time. I would hope that a player at 6'10" would get more rebounds than a player at 6'3" (not named Westbrook

). If not it is likely a result of the 6'10" player underperforming.
In the end, Mitchell is really carrying the team in the one area we needed it, on offense. Mitchell carries this load more than Simmons does, and does so very well. Their final TSA% was not that far off, 56% for Simmons, 54% for Mitchell, but Mitchell took many more shots from all over the court and was the primary focus of every defense they saw after the all-star break. Simmons was known not to be a scorer so opposing teams focused more on Embiid and the other threats, not so much Simmons other than to try to get the ball out of his hands. Mitchell can do real damage with the ball and without, Simmons really needs to be ball-dominant, so Mitchell is the more versatile offensive player. One more way to assess this, who do you want taking that final shot, DM or Simmons? Remember how that worked out for LeBron when he passed on the final shot instead of taking it? He learned his lesson and is far more likely to take the shot now. DM is that guy for us, not so much Simmons.
So their roles are different, their situations are different. The other question is, where would each team be without that player. The 6'ers would be in the playoffs, probably a lower seed but no way they miss it, just too much fire-power there even without Simmons. The Jazz? Very likely a lottery team without DM. I would also balk at the defense argument. I think DM is a strong 2-way player, even beyond steals. Simmons is probably a half a step above DM in that category, but it is not a stratospheric difference.
So if we give the award based on stats alone, Simmons is no doubt the winner. If it is meant to reward the rookie that contributed most to their team, then it is DM.