How do you be sure of that? I really doubt that one season's results serve as enough proof as to what is the better league. As Jebus said, was the Portuguese league the strongest back in '04?
I suppose we can agree that this season the Premiership teams have generally outperformed Spanish ones (although Spain looks very much on course to retain the UEFA Cup) however who's to say that next year it won't be the other way around with pretty much the same squads?
As I said, you have to look at results over a period of time wherever possible as opposed to one season's results. Otherwise you're basically saying that the Portuguese and French leagues really were the strongest in Europe in '04 (due to providing both finalists), even though it is widely accepted that they weren't and it was merely a one-off.