Yes, there is a difference between 10th and top for sure. But also, it's all about expectations. That's what matters, not league position per se. If your line of argument was correct then 10th would be less credible than 9th, 11th less credible than 10th, 12th less credible than 11th and so on. It doesn't work like that.
Anyone judging Wilder on his achievements this season would look at where they may have expected United to finish (roughly, i.e. top 2, top 6, top half, mid-table, bottom half, relegation zone) and judge progress on that - contrasted with the circumstances he's had to deal with.
There are other factors though as well.
Having employed literally hundreds of folks in my career, I would say the most important thing is how well the employer thinks they will fit in with them. If a football club is looking for a no-nonsense, tell it like it is, work on a shoe-string, get the fans onside, type manager then Wilder stands out a mile. If a football club is looking for someone with extensive Prem experience, or deep knowledge of the foreign game, international experience, etc., then Wilder wouldn't get a look in. So, that counts as well, hugely so.
Look, it's like this...
Wilder is a very good manager - but he won't be everyone's cup of tea. I can't see him being offered the job at Man City, Man Utd, Chelsea, Arsenal, Spurs, Liverpool anytime soon. But he could definitely be offered a West Brom or a Southampton or a Stoke type job, or some other "failing" Prem team ( a bit like what happened to Carlos the Jackal with Swansea).
Why would he be interested? He probably wouldn't, at this point in time. But give it until the end of this year/January transfer window - and if Wilder hasn't been backed and feels he can't take this club any further he'd be crazy not to jump ship if a bigger job came up. And I think he will. And I wouldn't blame him one bit.