The argument being made is one bad season in a management career of 6-7 years isn't the be all and end all in judging pedigree.
On the other hand, how many of them were genuinely good seasons? That's a serious question by the way, as I'm not sure how each of his Ajax seasons would actually be rated.
A fairly quick look into it::
17/18 - Joined Ajax halfway through the season, they improved a little but not enough to win the Eredivisie. Neutral season.
18/19 - Won the Eredivisie and got to the semi's of the CL. Great season.
19/20 - Didn't get out of their CL group and got knocked out of the Europa early. Was only on top of the Eredivisie on goal difference at the time the season was cancelled due to Covid. I'd say that was a bad season considering Ajax's advantages in the league.
20/21 - Dominated the Eredivisie, but got knocked out of the Europa at the quarters. Good season.
21/22 - Won the Eredivisie by two points, started the CL well by winning all their group games but then got knocked out by Benfica in the next round. Good season if we're being kind.
22/23 - Joined Man Utd and finished third in the league but was on the end of a few embarrassing scorelines and didn't win an away match against any remotely decent team. Won the League Cup, got to the final of the FA Cup, lost in the quarters of the Europa. Good season, certainly not 'very good'.
23/24 - Atrocious season, will very likely go down as Utd's worst in PL history.
So while he's been mostly good, it's not like he's been clear-cut great or even good up until now.
If there are any Ajax fans reading this, I would be interested to know if they'd rate those seasons similarly to how I have.