Isnt the Youth Cup an U18 tournament, so your gonna get mainly 17 year olds, maybe some 16.
The U20 South American tournament probably has mostly 19 year olds.
2 years can make a big difference in terms of what intensity a kid can bring in over 90 mins.
I think a more fair comparison to the U20 tournament is the U21 League at the very least, which is also mostly 19 year olds.
That being said, I partially agree that there is more "fight" in south america so the kids develop some steel.
English academy players can look a little bit like they are just out there to play a game for fun.