Where is this exception?In the history of Constitutions, no constitution explicitly states that violent speech is protected. Because until now, they've never had to...it's always been understood. Indeed, the the First Congress of the U.S. passed the Sedition Act just a few years after America was formed and it remained law until 1920 (when it was repealed by Congress).
For more recent examples, yelling "fire" in a crowded venue is also prohibited (this example is given to law students). It is important to understand that the law separates the content of the speech from the act it represents, so saying the word "fire" is allowed; what is prohibited is the act of doing something that will inflict harm on others by causing terror and mayhem.
As most constitutions are based on the U.S. Constitution, the U.S. history is relevant to Brazil because it demonstrates that no Constitution protects unfettered speech. To apply the above to what is happening in Brazil: the words are protected, but the act of promoting violence is prohibited. Thus, the far-right extremists' accounts are censored (because they are using those words to engage in the act of inciting violence) but everyone else can say what they said because other people aren't trying to incite violence when they quote the extremists.