It's called "attributeless damage" in Japanese amd should not be counting as magic damage. There are forms of damage reduction that affect all damage including attributeless damage though.
I guess I'm trying to figure out what it's set to on the server side. There are a set of bit flags that are either on or off. Each WS / Ability entry in the database would have a flags field that would look something like this.
Melee
Ranged
Magic
Physical
Breath
Slashing
Piercing
Blunt
H2H
Fire
ect..
As WS like Vorpal would have the flags Melee, Slashing and Physical set to 1 with the rest set to 0. Something like Drakesbane would have Melee, Piercing and Physical set to 1. Slugshot would have Ranged, Piercing and Physical set to 1. So on and so forth for every ability in the game. When the script runs to calculate out damage it does a series of checks against those bit flags to determine how to apply damage.
I'm about 99.99% positive there is a "non-elemental / attributeless" bit flag also and that Req would be set to Magical and non-elemental though meteor would also be set to that so there might also be additional flags to indicate WS/JA (Chapuli obviously checks these) and Spell (fools drink proves this).
I stumbled across things that receive 0 damage from Req.
http://wiki.bluegartr.com/bg/Apollyon_SE
4th floor mobs were completely immune to Req. Also I've done insane damage to Flan's and Slimes. So does MDA/MDT only apply on some mobs but not others? Dakuwaqa is a good example as I've not seen any reduced Req damage between 25 and 49% even though it takes less magic damage during that phase.
Again I'm talking about the fine grain mechanics which may shed light on times that Req might get reduced in damage.