Sounds like push may have come to shove.
It is up to the league, but I cannot imagine they will force other teams to go to Miami in these circumstances. I still wonder if there is a plan to simply shut down (for weeks, or perhaps the season) certain teams if the infections are too prolific.
If you start allowing teams to opt out of any "hot spot" before you know it you run out of places to play. Texas, California ... haven't been the best places to be for COVID as of late
Now clearly the Marlins as a team had an outbreak, but opting out on going to Miami has bigger implications than just the Marlins as a team.