Have you considered not all the funds are in multisig...not to state the obvious or anything.
They claim 97% of user funds. So that doesn't leave much. Is it likely that Coinbase's own funds constitute multiple % of all bitcoins... No.
I'd just assumed a narrow definition of "in circulation".
Yes that's their best bet now. Unfortunately in the same breath they'll have to reveal that Coinbase's holdings are not exactly substantial.