Kevlar is an organic polymer. If you read the description, the ‘kevlar’ is actually mutated skin patterned after the armour… I generally dislike that you get actual kevlar from them, it’s not meant to be the same stuff but something that resembles it.
When i designed them they were supposed to be one of three mutation pathways for soldiers, which would significantly reduce their frequency. It doesn’t appear that the “black ops” and “acid blaster” types are appearing in game though, and I’m not sure why.
As for being ‘bigger and badder’ than a hulk, they’re actually meant to compare more to a skeletal juggernaut, and are somewhat weaker than their bony counterparts in most regards. Plain jane hulks need some further evolutionary pathways, they’re a stage behind the juggernauts.