Nobody is saying they are.
The sat cannot generate a range vector from a simple vhf signal.
Who said anything about vectors? All Inmarsat has is one number per hour: the distance from its satellite to the airplane at that point in the hour. It has no direction, hence no vector.
Example: The satellite sends a ping message to the airplane. The airplane's reply arrives in 240 milliseconds (after subtracting out delays imposed by the electronics at either end). This means the airplane is 120 light-milliseconds distant, or 22,354 miles. Such a measurement would put the airplane right under the satellite, flying over the Indian Ocean.
Longer times would put locate the airplane on ever larger circles around the satellite's ground-track position. Then you apply other constraints to try to chop arcs out the circles. E.g., how far could have maximally flown in the time available? How come nobody up north or in Malaysia or Indonesia admits to seeing it on radar? Etc.
No, the sat doesn’t have an accurate distance to the aircraft.