There are some oddities with how Miami is being shown, but they skew the results in both directions.
Here's Miami zoomed in to the core, and with only tracts above 40,000 ppsm highlighted. Most of the densest tracts already hug the coast, and are quite a bit smaller than the typical neighborhood tract from New York or San Francisco, which skews them to read as "too" dense. On the other hand, there are some tracts that don't hug the coast, most notably Brickell Key and the coastal tracts north of 15th Street; but if they were reduced to hug the coast then they would be too small, so it's a tough problem.
Regardless, I think we can say with authority that Miami's high-rise coast is extremely dense, but that its neighborhoods are moderate.