yes it's true...but due to some treaty signed between Portugal and Brazil a few years ago, it's more easy for brazilians to ask for portuguese nationality than any other person from other countries outside EU, so if a brazilian player stays in Portugal long enought he can ask the portuguese nationality, beeing able to play any european club inside EU as an EU citizen. And this is a major advantage for brazilian players!
probably when many of them come to Portugal they have this idea in their minds, and this could be a good reason for having lots of brazilian players playing in our leagues...