The soul enchanter is able to apply enchants onto items that so not qualify for them; ie. feather falling on pants.
Playing on all the mods version 1.6.4 which contains eidolon version 0.2.7
elucent commented
It isn't documented anymore, but I believe the Apotheosis mod changes feather falling to be applicable to any armor piece. I can't replicate this behavior with Eidolon alone, so I believe this is just intended modpack behavior.