1. The Israelis have no incentive to target a hospital (since in the end, they’ll have to treat the patients when they retake Gaza) unless it was being used as a military base (e.g. guiding drones from the roof, which the Israelis didn’t claim).
2. The Americans said the Israelis didn’t do it, and Biden stuck his neck out to say so.
3. The Palestinians have a history of inferior weaponry, which is why even though they have been harassing the Israelis for years by sending rockets at civilian targets, they were seen as more of a nuisance than an existential threat (until they switch to ground assaults and started trying to retake territory that was on the Israeli side of the 1949 armistice (green) line. So the “friendly fire” explanation is credible.