The word isolationist (as in isolationism) applies to both to politics and economics:
: a policy of national isolation by abstention from alliances and other international political and economic relations
But if you're concerned only with economics, your own autarky seems like the best fit.
That aside, the most direct word is simply tradeless:
: having no trade
So, you could say:
They had a tradeless economy.
Something else (which is not a direct fit) would be embargo:
2 : a legal prohibition on commerce • a trade embargo
As it applies to your situation, you could say:
They were a completely embargoed nation.
Typically, a nation will specifically embargo a few other nations (if not just one), but there's no reason why a particular nation couldn't enforce its own embargo in relation to everybody. (Although I can't think of any instance when this has actually been done.) The word also has a sense of the temporary to it.