There are not significant limitations to scale other than the usual economic reasons. There are competing uses of many reservoirs, however - likely recreational

There are not significant limitations to scale other than the usual economic reasons. There are competing uses of many reservoirs, however - likely recreational boaters would strongly oppose taking over large sections of reservoirs, so floating solar is more likely going to be located in practice near to the dams / in existing keep-out zones. Yes, those claims are technically correct, in the same way that the claims we could power all of humanity's needs with a fraction of the sahara desert are also technically correct. In practice, competing needs for the water bodies and especially the cost of transmission lines / the lower cost of nearby desert land for many of the sites means that it's certainly an oversimplification.