The legs seem to do physical damage (can tell in eczak) so that makes a lot of sense why mage classes' mastery passives don't apply (as seen in Haku's bug thread)
Spatial Collapse:780% Damage | in video below it around 3.5b - 4b / 1 line -> Deal true Matt
True Arachnid Reflection:182% Damage | in video below it around 145M - 600M / 1 line -> Bug deal Att (i think it should work around 2-3b / 1line)