Unless that first thrust either broke his armbone, or severed most if not all tendons where it pierced, there's no reason for him to assume he's actually crippled. Either one would've been visible due to the reactions, tendons might've forced him to drop the glaive because his fingers let go, whereas the bone would've made the arm 'change shape'.
Admittedly, someone with a high pain threshold can use a broken arm as an improvised flail sort of, if they don't give a damn about it, likewise with severed tendons, but unless they tied or fixed the weapon in their grasp with something else, they would drop it most likely.
Not sure if the Ouhon just missed the tendons or if the guy had enough of them to not lose his grip and then swung with the muscles above the grip, which wouldn't have been affected.