It's a myth that all Argentina's beef is grass-fed. In the last decades, there is a huge shift to industrial mass production similar to other countries - which involves the typical methods to make an animal fat in short time, i.e., feedlots, restricted movements, ...
There are still farmers in the pampa which produce excellent meat using traditional methods, but they are nowadays in the minority and a lot of their meat (especially the prime cuts) is exported for obvious reasons. Bottom line is: if you get grass-fed beef here I'd always prefer it (tastes better, is more healthy and has better cooking qualities), but don't think just because there's the label 'made in Argentina' it has to come from the pampas where the happy cows jump around and eat grass before they get turned into steaks