They claim that their coins were stolen. The same way I could claim they are lying about this to FUD monero. They can't prove I am wrong and I can't prove they are wrong.
I haven't heard from any credible member who got his coins stolen from mymonero. Most often rather new accounts are claiming their monero got stolen.
What if a few trusted members of the community went fishing? We could all put small amounts of monero on the web wallet and see if we could get anyone reliable to confirm the thefts. Any other legendaries who are long time veterans of this thread interested?