"[T]he head of the IRS testified to Congress that there is in fact language in the [ACA] that contradicts [the] government's argument ... in King v. Burwell that the statute unambiguously authorizes the disputed taxes and subsidies in states with federal Exchanges.... [T]he IRS commissioner made this concession before the IRS rule had been challenged in court. The hearing was in August 2012, and the first challenge was filed in September 2012."