If Sling is marketing fully completed airplanes to the US using EE rules to skirt the normal rules simply so they can sell completed airplanes to buyers, it's going to be a tough sell with the feds.
BROKERING. Section 21.191(d) was not intended to allow for
the brokering or marketing of experimental aircraft. This
includes individuals who manufacture, import, or assemble
aircraft, and then apply for and receive experimental exhibition
airworthiness certificates, so they can sell the aircraft to
buyers.