Indeed he can't go long but he squandered plenty of chances to pass to the fullbacks in space because he was waiting for the forwards to press him, he also had a number of chances to thow out quickly and didn't
It's basically impossible to see on TV, since typically the fullbacks are high and wide enough that they're literally not on screen when Onana has the ball, so I'm not sure anyone not at Goodison can actually verify if this is true or not. It's not like McNeil and Harrison were pressing high up the pitch so this seems dubious to me.
Watching those comps of Onana passes last year, what stands out is because his team played a 3-5-2, they had 2 physical strikers (Dzeko or Lukaku are big and Lisandro is very strong and can hold it up when the ball is chest level or lower).
We don't set up like that, so all Onana can really do is pass it short most of the time and the benefits you get from his passing are:
1. Less hoofs out of bounds and 2nd balls in our half from rushed clearances.
2. Hope the other team presses up and he can hit some balls over the top like Ederson or Allison do against higher lines.
If the other team isn't pressing high, having a keeper who can really pass settles things down a bit and maybe gives the CBs and DM a better chance to make a good pass through the lines (because they get a better pass), but overall it's far less useful, particularly when Lisandro is out and we're playing a right-footer on the left.
Onana almost had a hockey (pass before the assist) to Mainoo early in the game but he slightly overcooked the pass to Garnacho.
It's for some reason not on this clip:
but there's nothing really wrong here and a good pass to Dalot and a solid one to Rashford too, and as you can see the Everton wingers aren't really even on screen they're so deep and wide, it's Doucoure and Calvert-Lewin vs Onana, Maguire, Lindelof and Mainoo and Onana mostly seems to make the only choice he can while looking for Mainoo whenever he's free (which is preferable to just giving it to a CB).
The scary part of the Onana signing is that he seems pretty average as a shot-stopper (made some good saves lately, but his rebound control is slightly below-average for a Prem keeper and he's not De Gea in terms of miracle saves) and he cost a lot of money, but unless you can sign a top 3 keeper in the world like Liverpool did, or find one for cheap, paying 50M for a very good keeper with presumably 5-6 years left is fine. We need to modernize our football, Onana will help in that regard.
I don't know why people are mad that he takes his time on the ball trying to draw a striker or #10 towards him, that helps the CBs bring the ball out when he passes wide of them, and what's the alternative, he kicks it long. Doing that, hitting a fullback in stride once per half and the DM say twice per half and being able to hit Hojlund consistently when everyone is marked and the pass to a CB isn't on, maybe 4-5 balls over the top to Garnacho, Hojlund or Rashford against a high line over the whole season and another 4 or 5 straight to a #8/#10 like Bruno when the other team leaves a gap, and only say 1 bad passing mistake leading to a goal per season and that's what a top passing keeper looks like to me.