The video claims that that specific missile didn't cause the explosion. That is not mutually exclusive to IDF's claim of failed rocket launch.
Watch about around 15 seconds of this video. Failed launch can look very different from normal missile attacks. Their claim, whether true or not, has very little real evidence and jumps to conclusion.
Also worth noting that aljazeera is run by Quatari government.
They have long been criticized for the antisemitic, anti-israel sentiment and Islamist position, which is very much expected. I have nothing actively against them. It's just that they are Fox news equivalent when it comes to Israel-Palestine situation.
Edit 2 (a day later): AP did their own video analysis and came to slightly different conclusions: https://lemmy.world/post/7113210
Tl;dw: Decent video evidence shows that the rocket the IDF claims hit the hospital was in fact intercepted and destroyed by the iron dome missle defense system. Thus it's unlikely the hospital was hit by an errant missle fired by Hamas or other Palestinian groups.
Edit 1: If there was another missile, wouldn't it be seen in the video like the other missiles?