Law isn’t defined just by legislation, it is also defined by case law. A judge’s ruling on a previous case makes that ruling law.
Now, I’m not saying this ruling is appropriate - I simply don’t know enough about how it came to be. But if Brazil made laws about social media companies and then a judge made a ruling based on that law requiring social media companies have a representative, then that absolutely is valid law.
To draw an example, the EU never made a law about cookie splash screens. The EU made GDPR law (well, strictly speaking they made a directive, then member states make laws that must meet or exceed that directive), and then a judge interpreted that law and made it a requirement to have cookie splash screens. I would personally argue that the judge was trying to shove a square peg through a round hole there, when really he should have identified that data collection is in fact a secondary transaction hidden in the fine print (rather than an exchange of data for access to the service, this isn’t how the deal is presented to the user; the service is offered free of charge but the fine print says your data is surrendered free of charge), and he should have made it such that users get paid for the data that’s being collected. However, the judge’s ruling stands as law now.