My dentist said something similar, that flossing will pull a bunch of stuff out of my teeth. I never floss and on the rare occasion I do nothing comes out, maybe a little blood. I don't know if anyone else has experienced this, I don't really floss anymore though as it just feels like jamming string between my teeth for no reason. Never had a cavity or dental problem of any kind, in my mid 30s. Floss once a month maybe.