Giovanni van Bronckhorst won the league with Feyenoord. So does that mean it is a huge achievement if a patently average manager can do it, for me less so. That said, it is more of an unfancied club than Ajax and PSV, yeah.
I think what Amorim is doing at Sporting is more impressive and difficult, but clearly there was a misalignment there. Slot seems like the second option to me, but targeting both of them makes sense. The idea is similar, which is that if you can win at clubs who arent the best and biggest in your league, you may have the traits to be an extraordinary manager who can do things against the odds, etc...
So we are banking that Slot is an extraordinary manager and it s possible he is, but again, I dont think a single person can give you any certainties about being good in the Dutch League and what that means. It is an awful league.