Justicebringer 3000 Specs have a dynamic primary stat, they have both Strength and Intellect but only use one depending on class/spec.
Holy Paladins for example use Plate Intellect gear so it’s certainly not useless 
If a warrior or death knight were to look at that item, they’d see it as Plate with Strength on it.
The only requirement on those for a plate wearer is having the appropriate level of Engineering to use them.