Their reaction is priceless. Reddit user “Caucasian_Male” provides a bit of insight.
Porto Alegre is the southernmost capital of Brazil, and is very close to Argentina. In many ways its culture is closer to Argentina than to Brazil, to the point that people from the city (and its state) and Argentinians call one another “hermanos”; spanish for “brothers”. You can read this word repeatedly in the YouTube comments. Moreover, we denizens of Porto Alegre are used to Argentinian presence in our city.
There are two big rival soccer teams based in Porto Alegre: Internacional and Grêmio. Many supporters of Grêmio regard themselves as very closely related to Argentinians. As an opposition, some supporters of Internacional deny this association to themselves. As consequence, the Argentinian soccer team is associated to Gremio and dissociated from Internacional.
This party has apparently started as a provocation from three Internacional supporters. During the course of the video, however, when it becomes a celebration, you can notice they join in.
-Disgruntled Sports Fan