I'm thinking the hinge and other stuff under the forearm is to show that the hatchet is a component mounted to the arm, not something merely carried as most existing art suggests.
In the public comments on his Patreon, Anthony Scroggins has said:
"Nah I haven't designed it to let go of and stow the hatchet. The piston style attachment just helps with leverage and stability ... it's clearly not a drop and pickup system."
So its not to suggest the hatchet is retractable apparently, just that it isn't droppable, and to provide an indication the hand is reinforced enough to handle the weapon.