Short version that doesn't require technical knowledge is Red Bull Racing (RBR) were using a system on their brakes that wasn't allowed but it was suspected that teams were using anyway. The rules were publicly clarified before the Miami GP. Ever since then Max Verstappen and Red Bull in general have started to lose pace compared to other teams, breaking Max's long run of victories. The tweet is suggesting that Max's break issues that caused him to DNF in the Australian GP were due to a fault in this system and the loss of pace is due to it's removal.
Edit to correct detail that I mis-remembered. Max did not DNF in Miami, in fact he finished 2nd. It was Australia that he retired from.
By my understanding, it was more of a grey area. It seemed like the system was technically allowed according to the wording of the rule but clearly against the spirit of the rule. Prior to Miami, the FIA made it clear that they would interpret things like this as a breach of the rules, effectively allowing teams to remove them without punishment before they were caught.