Jost and the government claim that there is an ambiguity or absurdity and that legislative history must be looked at. But the legislative history does not help them: the tax credits available to those who purchased through a state exchange were intended as an inducement to states to establish exchanges, the did not anticipate that many states would not establish exchanges.
Michael Cannon has assembled all the court filings and opinions, and analyzes and refutes Jost’s claims, available here:
Since many states did not establish exchanges many (most) people do not have tax credits available. This causes the cost to be too great for many people. To remedy that the IRS illegally gave tax credits (subsidies) to those who purchased through a federal exchange.