What exactly does a visa agency mean by the above statement?

 If you don't have the required 800K, the visa agency provides the funds just long enough to satisfy immigration, and you will probably have to pay the agency around 20K for the privilege, of which immigration gets their cut. Just another loophole for those with insufficient funds to meet the requirements.

It means they loan you the money at a high rate and interest to put in the bank to apply for a dodgy extension at Immigration.