The implication of that is that consumers, not the oil and coal companies, were the primary beneficiaries of externalizing the costs of pollution over the past decades, in the form of having access to cheap fuel.
Put another way: Exxon last year had revenues of $240 billion, and profits of about $20 billion. If allowing pollution to be externalized means that gasoline costs half of what it should, that means Exxon's customers saved $240 billion by pollution the environment. That's more than Exxon's profits in an entire decade.