What is market value other than "the community's valuation of the external costs and benefits of the activity"?
External costs and benefits are, by definition, costs and benefits that are external to the market value. If they were within the market value, they would have been internalized, and therefore not be externalities.
So the answer to this rhetorical question is: exactly not that. Or perhaps more precisely: don't talk bollocks.
This guy (Mark Wadsworth) is a free-marketeer, so I am embarrassed to see him displaying such fundamental lack of economics, or even basic logic. But it's typical of the crowd that support Land-Value Taxation.
He must work in financial services.