That doesn't strike me as fitting the requirements. They need high transaction rates and true high availability (don't know if Bigtable is as good). And they don't need petabyte scaling, which I gather is one of Bigtable's big attractions.
You have a good point. You are kind of implying that it is good for transactions in large batches, but may not have the 300 ms response.
But the oracle RAC (just guessing here) may act as a cache between the 300 ms part and the actual tables, and to be a fat reliable pipe.
But they do take on some interesting problems, such as how do you keep this up across system upgrades, hardware failures, application version migration, among others.
... not any more!