Ah - I see what has happened. My source was quoting from the Wiki page on the Oxygen Cycle, where the figure for chemical weathering is given as 50 x 10[sup]10[/sup] kilograms; he wrote this as 50 x 10E10kg, which I converted to 5 x 10E11. It should be 5 x 1E11.
For a minute there I thought he mean 5 x 1E12.
This figure for chemical weathering does seem a bit low. The mass of atmospheric oxygen is a bit more than 1 x 1e18kg (I checked that several times, and it should be correct) so it would take 2 million years or so to remove all the oxygen by chemical weathering alone. Even I thought it would be faster than that.
Really, it’s not the oxygen content of the atmosphere you have to worry about - its the carbon dioxide. Oxidising the biosphere would raise the CO2 content to unprecedented levels, which would melt the methane clathrates and cause the sort of extreme global warming that environmentalists are worried about. Without a biosphere the planet would, indeed, be uninhabitable, so Mr Dibble is correct- but not quite for the reasons he gave.