If those 3.3m people drive down to Heathrow, they will presumably generate an out and return leg, so that would be 6.6m pax, which is indeed very close to 10% of LHR capacity.
6.6 million sounds very high, in fact even 3.3m does. CAA stats show the West Midlands accounting for around 2.5% of Heathrow originating traffic (6.6m equates to almost 15%).
But whatever the figure, it's hard to see how that would support a whole bunch of new direct services from BHX, which is what's being proposed.