That depends on which NHC forecast you look at. They were moving the track on and off Cuba, several times. On the Thurday, when they first started talking about a Cat 4/5 hitting Miami, they were showing it brushing the coast of Cuba, but NOT making landfall there... hence, little to no weakening.
http://www.nhc.noaa.gov/archive/2017/IRMA_graphics.php?product=5day_cone_with_line_and_wind
Thanks for that graphic. It shows they forecast a Cuba hit as early as Monday. By Wed they had a Cuba miss and a hit just to the west of Miami. By Friday the forecast was a Cuba hit, still uncomfortably close to Miami, but with a Cuba hit the strength is unknowable. By Friday it was all but over.