How it works - Flightradar24.com - Live flight tracker!
"The primary technology that Flightradar24 use to receive flight information is called automatic dependent surveillance-broadcast (ADS-B). Aircraft gets its location from a GPS navigation source (satellite). The ADS-B transponder on aircraft transmits signal containing the location (and much more)."
Implication is that the altitude and groundspeed are from GPS. No air data.
Next question is the location of the GPS antenna, ADS-B antenna, GPS and ADS-B avionics on this airframe.