The speed buff on phoenix cloak is fairly recent, there was a final hox revamp in 2014 which reduced phoenix cloak duration, added the movement speed feats and increased the damage deflection while removing ANY penalty. The interrupt mechanics are even older than that indeed. That is also when they added the KB immunity in demon form (or made it stronger at least).
PC used to be 30s duration, 30 deflect, -100 fire damage, -20% cold protection and -10% movement speed or something like that. IMO that was actually much more interesting than current version.