I do not know about his involvement with Sambo. But it is probably true. An Olympic level wrestler would easily have access to Sambo in Russia. In Russia, few people walk right on into Sambo. They normally come from a Judo/Wrestling background. Yes he boxed a bit in his teens but that was more for diversity. Or so I have been told.
My coach told me a story from Alexander Karelin's routine. He met Alexander Karelin when he trained overseas. But I'm not sure if he was at the Sydney 2000 Olympics. A few wrestlers got together and started talking. He used to pick up a huge, heavy fridge and carry it up 9 flights of staris to his apartment. Now I do mean a proper full size fridge. One for a typical family home, not a mini bar fridge.
Apparently, that was his strength training routine. Between that and live wrestling Karelin said he had no need to lift weights in a conventional sense. By that, I mean dumbells or barbells.
The guy was really flexible too. My coach Karelin had the flexibility of a dancer. He could stand in a doorway and put his foot into the upper corner. Impressive to watch.