Horsemen's Aid in Rider of the Apocalypse Hero talent tree works in half efficiency.
The tooltip states that
"While at your aid, the Horsemen will occasionally cast Anti-Magic Shell on you and themselves at 80% effectiveness.
You may only benefit from this effect every 45 sec."
The player may benefit from it every 45 seconds, but it states "and themselves". I've never seen a single horseman under Anti-Magic Shell spell at all.
They do not cast it on themselves, but they should.
The talent should work something like this:
If the Horsemen's AMS procs and player didn’t benefit from it in 45s duration - they cast it on player and themselves.
If the Horsemen's AMS procs and the player benefited from it in the last 45 seconds - they cast it only on themselves.
The process can be seen at this video: https://youtu.be/ztgSSb6XKO4?si=QJUGBYxjcYzxzz95&t=82
Time range - from 1:22 to 1:38.