They do infact claim to store (or at that time have stored) 10% of all mined bitcoins... What an utter lie.

Charlie Lee just confirmed it:

https://www.reddit.com/r/Bitcoin/comments/4md665/coinbase_may_have_lied_about_the_number_of/d3uyda7


Here is a screenshot in case they change the comment again.
http://imgur.com/wCu58Qz