Dumfries replaced Santiago Arias who went to Atletico and initially was a bit raw. Positioning and things that come with experience were a question mark. He has a tremendous frame for a RB though - big, strong, pretty quick. He had a better season last year but I think this tournament is showing that he's ready for the next step.
I wouldn't buy any player based on tournament performance. IF he didn't impress in the league, then no.