Viewing these words as a mistake is the simplest interpretation of the law. The other option is to re-interpret lots of other sections, and change the law to be at odds with how the people writing it meant for it to be interpreted.
The writers of the law clearly wanted to establish state exchanges for any state that wanted them, and a federal exchange for any state that didn't want to roll its own, and that all of these exchanges do the same thing. This might not be apparent in that little snippet, but it's very much apparent in the text of the law itself.
It's not as though the SCOTUS majority is pulling meaning out of nowhere for just this passage. Quite the contrary, they'd have had to re-interpret a lot of text to infer that the law was written so as to exclude subsidies for the federal exchange.