It uses another Tegra chip exploit. There was a reason why cfw was stuck for 7.0 it was due the inability to actually get the keys required to boot the switch into horizon. (Nintendo made significant changes to how pk1 and pk2 was handled)
And sept was able to dumb those keys. There was a reason why SX OS was having a hard time getting to 7.0. It was because they couldn't get the keys, and to avoid using sept, they waited for someone to drop they keys so they could just bake it into the loader.
Second off your wrong about "release"
Again SX OS's code was based off a significantly older build of atmosphere. So in reality if you seriously want to argue that because there was no release version, that SX OS was first, then sx os is not a release but a alpha build of atmosphere that was closed sourced, and the only change that was added was emulating game carts.