From FCCs doc that the other counter linked > Radar altimeters used in civil and commercial aviation operate using either unmodulated pulse radar or frequency

From FCCs doc that the other counter linked > Radar altimeters used in civil and commercial aviation operate using either unmodulated pulse radar or frequency-modulated continuous wave (FMCW) radar technology, with FMCW being far more common among models developed in the last few decades. In either case, the above ground level (AGL) altitude of the aircraft is measured in the radar altimeter by transmitting radio frequency (RF) energy down to the ground and receiving a portion of this energy back through reflection off of the terrain or other obstacles, and determining the round-trip propagation time of the RF energy. The radiated power levels are low, typically on the order of one watt, and thus highly sensitive receivers are required for radar altimeters to function properly. As such, radar altimeters are highly susceptible to RF interference entering the receiver, which can negatively impact their performance. The planes use the band to bounce signals off of the ground and that needs to be super sensitive. That's why they probably can't afford to properly filter within the 200 MHz guard band.