Immediately before the definition section, Wikipedia references Kármán's abstract concept from his autobiography:
In the final chapter of his autobiography Kármán addresses the issue of the edge of outer space:
"Where space begins… can actually be determined by the speed of the space vehicle and its altitude above the earth. Consider, for instance, the record flight of Captain Iven Carl Kincheloe Jr. in an X-2 rocket plane. Kincheloe flew 2000 miles per hour (3,200 km/h) at 126,000 feet (38,500 m), or 24 miles up. At this altitude and speed, aerodynamic lift still carries 98 per cent of the weight of the plane, and only two per cent is carried by centrifugal force, or Kepler Force, as space scientists call it. But at 300,000 feet (91,440 m) or 57 miles up, this relationship is reversed because there is no longer any air to contribute lift: only centrifugal force prevails. This is certainly a physical boundary, where aerodynamics stops and astronautics begins, and so I thought why should it not also be a jurisdictional boundary? Haley has kindly called it the Kármán Jurisdictional Line. Below this line space belongs to each country. Above this level there would be free space."
So Kármán presented his concept of the crossover point between aerodynamic lift and "Kepler force"; Andrew Haley referred to it as the Kármán Line. Since the exact crossover point will vary with the particular design of an aircraft, Kármán's 91.44 km figure is not actually definitional; FAI rounds it to 100km, which is the Kármán line for some purely notional aircraft.
Furthermore, section 2b of the FAI article you link does in fact refer to this same definition:
In Aeronautics, level flying higher and higher meant to deal with less and less dense atmosphere, thus to the need of greater and greater speeds to have the flying machine controllable by aerodynamic forces. A speed so big in fact, that, above a certain altitude, could be close or even bigger than the circular orbital speed at that altitude (i.e. lift was no longer needed, since centrifugal force took over; and consequently aerodynamic flight was meaningless).
So both the articles provide what you claim is lacking.