You're the one arguing that there's some special exemption for aircraft, but have done nothing to substantiate that claim.
Besides, with the GDPR it works the opposite way. You have to justify why your data processing is legal, not the other way around.
And for fucks sake, neither of Flightradar24 or ADSBExchange even offer a GDPR-compliant privacy policy. ADSBexchange does not offer one at all.