So the internal SSD is SATA-based? Thought it would've used a different interface for the sake of increased speed.
I think the article is simply saying that it uses the latest Samsung MHX Flash controller, not that it's using USB-to-SATA internally.
The MHX has three Cortex-R cores. One for reads. One for writes. One for external I/O.
So Samsung most likely programmed the I/O core to talk to the host computer via USB, instead of via a SATA interface.