It all depends on what the meaning of “by” is.
The four liberal justices clearly believe that an exchange established “for” or “in” a state by the federal government is the same as an exchange “established by the state,” to quote the relevant statute. Justices Scalia and Alito (and presumably the silent Thomas) equally firmly believe that words mean what they say.
So this case, as expected, turns on the views of Chief Justice Roberts and Justice Kennedy, who gave very little away at oral argument. If the government wins here, then not only will Obamacare continue to be rewritten by the IRS, but any executive agency – and any future president – will be able to rewrite any law. Accordingly, for the sake of the rule of law, I fervently hope that Roberts and Kennedy decide to enforce the Affordable Care Act as written and let Congress clean up its own mess.