There’s no doubt that the incident during the Copa Libertadores qualifier between Grêmio and LDU was serious, but the article’s attempt to link it to the World Cup is erroneous.
Because the Arena do Grêmio will NOT be used at either the Confederations or World Cup.
Unfortunately, it seems that any incident in Brazil (Santa Maria, Grêmio, violence in Santa Catarina / São Paulo), occurring between now and 2014, is going to be used as a stick to beat the country with about its readiness for the World Cup. Whilst a lot of this criticism is justified (infrastructure / stadium delays, corruption, etc), putting out poorly researched and false stories to whip up the anti-Brazil sentiment is pretty feeble.
A quick search by the PA journalist who wrote this story (even on bloody Wikipedia) would have highlighted that this stadium will not be used in 2014. The fact that the sports-desks at the Guardian, Independent and ESPN published it on their sites without even noticing the error, or checking it themselves, points to either willful ignorance or a shoddy lack of knowledge about 2014.
An hour ago (10.30am Brazil time) I wrote emails to all the sites concerned requesting that they correct the error. They are as yet to do so.