They may be utilizing cold wallets. That doesn't necessarily indicate insolvency.
Also, that address holds only their Bitcoins whereas most trading volume on Polo is in ALT coins.
I stopped using Polo though for a different reason, because I got tired of them always putting hour(s) long holds on my withdrawls.
Hours long holds on withdrawals is a warning sign for me to start moving your coins out of the exchange. What reason do they have to start holding our withdrawals for that long? Poloniex is also attested as very slow and lags in situations of very high trading volumes. Many traders are already losing money by using that exchange for the last 2 weeks.