It just means that it's profitable for them as a company to make it at that price, but their volumes are still relatively tiny compared to the overall fuel market. Presumably were we all to switch to gas-to-liquids to increased demand on liquid natural gas supplies would push the price up quite significantly. So it's not likely that we could replace oil with a $20 a barrel substitute, but it's plausible that coal-to-liquids and gas-to-liquids could serve as relatively affordable substitutes even on massive scales.

And I'm not sure how much gas there is, but there's enough coal to last for 50-140 years, depending on demand.