You're missing the point. If the manager of Celtic won the Champions League twice, despite how poor his league is, I'd say he's a top manager because he's won a competition where Europes elite clubs compete. He's not just a bug fish in a small pond.
Also, the Prem is much better than what it was 10 years ago but Fergie is still winning even though there are now 3 or 4 teams capable of winning the league.