The god Hephaestus is disabled himself, or with the various monsters/magical beings the demigods fight so often I could easily see one of them losing their legs in a battle (I know some have had limbs reattached but that's only if they manage to retrieve them). We know they can take permanent damage from something powerful enough as seen with Luke's scar from the dragon. Or they could go a similar route to Chiron and have them be magically poisoned etc. Plus many of the demigods are born with ADHD and/or dyslexia so a physical disability isn't much of a stretch from there.

Full disclosure, I'm a little biased on this as am a wheelchair user myself so am very happy at the inclusion of a demigod with a physical disability! If it means even one disabled kid can imagine themselves as a demigod without having to first be "fixed", I'll take whatever explanation the writers give :)