The latter. An important clue is the fact that there is no coefficient estimated for Adidas.
Again, I’d encourage you to think more broadly than what the “significance” is in reference to. The fundamental question is not about the significance of the coefficient, but rather about the meaning of the coefficient, irrespective of what its posterior distribution is and whether or not that distribution includes substantial probability mass on both sides of zero.
In your model, the parameter for Nike means the expected difference between Nike and Adidas with all other covariates held constant. And the 95% credible interval for this parameter that brms has constructed for you doesn’t overlap zero.