As it is well-known, Komodo dragons today prey mainly on deer, water buffalo, pigs, goats and horses as adults. However, these have only been present in their range for 4-5,000 years at most. And even ignoring the fact that the idea that Komodo dragons evolved gigantism to prey on pygmy stegodons is disproven (though they likely preyed on them anyway, they were already giant when they reached Indonesia), the pygmy stegodons lived during the Middle Pleistocene, meaning there were hundreds of thousands of years between the extinction of the pygmy stegodons and the introduction of mammalian megafauna by humans. So, what were Komodo dragons eating between those times? I think I heard David Attenborough say in a documentary that they cannibalized younger members of their species, but living on this alone violates the laws of thermodynamics.
There wasn't "hundreds of thousands of years" between stegodon extinction on Flores and the introduction of Asian ungulates. The stegodons became extinct around 17,000 years ago at the latest, along with a number of other mammals and birds. Volcanic eruptions are one suggestion as to cause. There are substantial ash deposits between the last remains of the extinct species and the earliest evidence of modern humans on the island. However, there are few archaeological sites on Flores so there is some extrapolation. It still (potentially) means there is a gap of several thousand years with no large-bodied mammals on the island. Nobody knows what the dragons would have been feeding on, but there are other animals there such as rodents, bats, megapodes, other lizards, pythons, etc.
I agree, and in addition there would have been tideline scavenging of carcasses large and small. Before the collapse of whale stocks cetacean carcasses were much more frequent on the world's beaches - it is probably why the California Condor managed to survive for example. I am not sure what the local whale and dolphin species are, but even one or two beached each year would easily have kept a population going.
