"Transit" or "In-transit" traffic refers to passengers who arrive and depart on the same aircraft within 24 hours."
By "the same aircraft", they actually mean "the same
flight".
Otherwise it would include anyone who arrived on a morning flight from, say, Edinburgh or Leeds and returned home on an evening flight that happened to be operated by the same aircraft - that's obviously not the intended meaning.
As far as I'm aware, there are only two flights that currently deposit transit passengers at Heathrow: the 3-times-weekly KU101 which flies Kuwait-LHR-JFK with a 2 hour stop and KU102 in the opposite direction, so the number of transit passengers per year must be very low.