My concern is that if the overnight stay is official policy (not just a ploy for cash), then the ETA could conceivably generate an entry in a database that they can't override (to allow you to leave the same day). Unlikely, perhaps, but possible.
Malaysian borders have a history of insisting you spend one night. Returning on the same day has typically required a bribe, and this ETA is an extra wrinkle.
Private clinics/hospitals here will pretty much always try to sell the most expensive thing possible, whether you need it or not. Even individual doctors are incentivized to do so. Question everything and DYOR.