They were the reigning Olympic champions.
Yes, Uruguay were the then current Olympic Games Football Gold Medalists and unofficial World Champions.
FIFA World Cup Finals 1930 Uruguay
At the same world cup?
Pele must have won both.
Having read up on this last week, it is fresh in my memory (so if I get it wrong, my memory really is as bad as I've wondered lately)
Belgium
France
Roumania
Yugoslavia
........ and none of them asked to go, but had to be persuaded even after extension of deadline.
What surprised me was 1st hat-trick .... USA, 3rd place USA
1966