I'm struggling to understand how the wind can contribute to TAS on the ground (and shorten takeoff) and then once in the air have no effect on it?
That is because while on the ground, the aircraft is "connected" to the ground by friction.
So a stationary aircraft on the ground with a 50 kt headwind will have a 50 kt TAS. A stationary aircraft with a 50 kt crosswind will have 0 kt TAS.
Once in the air, the aircraft is free to move with the air mass so the surface wind speed is irrelevant to the aerodynamics of the aircraft.
A different (but important) issue entirely, is that surface wind is critical to the pilot's
perception of air speed. A dangerous situation can arise when an aircraft is turning downwind at low height with a strong surface wind. The ground speed will increase which the pilot may interpret as an increase in TAS and pitch up/throttle down to compensate, leading to a stall.