Their argument is legitimate and made in good faith.
Looks like it can be done.
There’s more than a scintilla: there are the actual words that Congress passed.
And you’re completely incorrect in your claim that anything in that link “explicitly” says that fed exchanges get tax credits. The link points out that 1321(c) requires federal exchanges report to the IRS and to the taxpayer any information regarding tax credits provided to individuals through the exchange. From this one might infer that Congress intended the federal exchanges to offer subsidies, or one could infer that Congress left that in as a placeholder, intending the report to constitute “zero dollars,” unless Congress acted further to authorize subsidies for federal exchanges, which they never did.
Do you not understand what the word “explicit” means?