You are quite right for the cellular antennas radiating lobes.
However, the reception cone for radar altimeter antennas is quite large : between 35° and 60° of cone aperture for the 3dB (half power) cone.
Here is a datasheet example of such an antenna ; it announces a minimum of 45° in roll angle and 40° in pitch.
https://www.sensorantennas.com/wp-co...7-2002-121.pdf
So, basically, a plane which is turning final with a 25° bank angle at 2000 ft height is sensitive to radio beams coming from more than 1.7 km or 1 sm from the outer direction of the turn (2000 ft x tan(45°+25°)).
And if we accept a gain loss of 10dBi, it probably extends to several statute miles away.