From what I learned while searching for testing options while in Orlando, Advent Health is the official health care provider for Disney. If it's in partnership with the airport, it must be legitimate and the only issue I see would be how long does it take for results.
It is certainly more legitimate than some of the services we explored due to being unable to find an empty time slot at Walgreen's or CVS who are providing free testing (looking at YOU, Covid Clinic - AVOID).
In the end we used Beeper MD - they saved our bacon when I miscalculated how far in advance to book a test with the pharmacies AND they came right to our room at Beach Club AND it was totally free AND we had our results within 28 hours.