"Reef safe" is not an all or nothing denomination. There are many people with sps reef tanks that have large angels with no impact. Many fish generally considered not reef safe can be kept in reef tanks as long as the corals they bother are kept out of your tank.
With regal angels specifically, they are generally fine with sps and clams, but have a taste for zoanthids. Before getting my regal all but a few of the zoanthids I couldn't get to were removed from the system and placed in another tank. Interestingly, within four hours all of the remaining zoanthids eaten by the regal, with all of my sps, clams, ricordia, and fungia left alone.
Many large angels are low risk in sps tanks, including regals, emperors, and asfurs... the only angels 100% reef safe are those in the genus Genicanthus...