QF actually DO call themselves "THE", but only on 123.45 or guard.
It's a shorthand thing. Instead of me saying "this is the Brisbane-Singapore daily", I say this "the QF51" or "the 51". QF pilots generally know where the callsigns are going to.
And yes I agree that any conversation on guard apart from trying to get an aircraft to go to 123.45 is unprofessional.