The biggest difference has been the 3rd down defense. After having a top 5 unit on 3rd down in 2020, Miami fell to to the bottom 5 for the 1st eight games of 2021. Apparently Boyer was deploying more zone coverage on 3rd down, which isn't the strong point of the DB's. Man coverage is the stronger aspect of the secondary and Flores went away from that early on.
Since they've changed back to the man coverage schemes on 3rd down, the entire defense, has been able to play aggressively.
Also the zero cover scheme was being tweaked and expanded on from what was deployed in 2020. It took some time for the players to adjust and may be the reason why it wasn't used earlier in the season.
Miami's defense with the current players is at its best in man coverage with multiple looks up front. Not sure of the reasoning why it took so long to utilize, this year. Over the last 3 seasons the defense carried the team, using this scheme and coverage.
Miami can become dominant based just on its current defense play. They have all the elements to succeed. They ain't perfect by any means, because the still seem susceptible to breakdowns in communication and run defense. But the aggressive, ball hawking play is back, looking better than ever.