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.
I have been very lax on security measures with a few mymonero wallets just to see if someone is gonna steal them. Nothing got stolen in the past 2 years.
The main takeaway is, don't trust webwallets with any coin. It is not like people ever stated that webwallets are secure. It is just reckless behaviour on the side of the user to use them anyway . There are more secure alternatives available.