This stat is relatively meaningless, especially early in the season.
Cardinals are 9-14. If they had swept the Sox, instead of the other way around, you could say the Sox were 2-9 against teams above .500. If BAL swept the Sox, they'd be above .500. If the Sox beat TOR one more time, they would be a below .500 team.
If the Sox were beating up on all these teams, they opponents would mostly be below .500 and you could say "they haven't played anybody." If the Sox were doing worse than they are now, they'd have an even worse record against teams above .500. It's just a function of the season being only a few weeks long.