There is actually a theory of legislative intent that makes the limitation of subsidies to state-established exchanges not an error, but a deliberate policy: That Congress did this with the aim of placing state legislatures under pressure to establish exchanges (at a time when it looked as if many legislatures might be recalcitrant) by threatening them with the hardship of the people of their states having to do without subsidies if state exchanges were not instituted. That is, that it was a form of legislative arm-twisting.

This does gain some credence from the explicit provision of the original ACA that states were to expland eligibility for Medicaid, and that any state that did not do so would lose all Medicaid funding—not merely not get federal aid with the increased Medicaid expenses (which would have been perfectly sensible if the states didn't take on increased expenses!) but not get any further funding at all. That provision was struck down by the Roberts court in an earlier year as impermissibly coercive. But where there was one provision with coercive effect, there could have been another.

And the very "untold chaos" to which you refer would surely have caused enough hardship to make it a plausible threat, in the spirit of "Nice health care system you got here; be a shame if something happened to it."