It effectively is. The subsidies are paid right out of the US Treasury and go directly to insurance companies. It is single payer of an enormous subsidy to the Health Insurance industry.
But the same subsidies exist, from the same source, in the state exchanges. What is the difference? Migrating to a different platform to buy the same insurance with the same subsidy is not an ADDITIONAL step toward a single payer.
The step that is Obamacare in the first place was already a big one, though, that is for sure.