I can confirm that once the prereqs are meant in the dev kit (specialist ammunition 2, steel tools, bow) it can be unlocked via standard feat points.
I just am not sure if that was intended (I haven’t been paying attention as far as where to unlock the new stuff) or if its mean to be unlocked via what Barnes describes (needing scrolls or whatever).
When you kill the boss, you receive a Skeleton Key, a Fragment of Power and a Parchment Note. When you use the note, you unlock Specialist Ammunition IV. I am certain the tree is a mistake.
Edit. Confirmed: Shrouded Child of Jhil drops SCROLL to unlock Specialist Ammunition IV.
This isn’t like errant Serpent-Man sets being craftable by T4 thralls, this is the OP ranged armor-melting killer be-all weapon that PvP (and PvE I’m hearing) players should and can preventatively wall off.