They’re not saying it’s a fragment from the iron dome stopping the missile, which happens over the desert, they’re saying it’s most likely a failed rocket launch that went off course.
To argue that the Israelis were able to react so quickly and fire a JDAM at the hospital in response to and while the rockets were firing is itself a stretch.
All of the available information points to it being fired from within Gaza. It’s only the armchair generals who claim to be able to identify a rocket by the sound (despite having never actually witnessed it themselves) who say otherwise.