Describing the report as credible is not accurate. Given that many news outlets retracted their initial claims and the official statements, it is very likely that there is enough evidence that Israel did not bomb the hospital and that the reported number of casualties is inaccurate. You using the "fog of war" argument to dismiss his claim—which was honest enough to say "not certainly"—is irrelevant. You could say this about almost every other reported event in Gaza.
If you include the surrounding context, that al Ahli had been targeted before, and since, and that other hospitals had also been not only targeted but actively sieged for days, at is in fact credible that the Israeli military targeted and hit the Hospital.
Now remote forensics on the site makes it implausible that the initial reports of an Israeli airstrike were true, however we still haven’t ruled out other types of munitions by the Israeli military.
Note that the initial reports of those supportive of Israel were also false. They claimed that they captured the rockets which they claimed hit the hospital on camera. It turned out this footage was of an unrelated rocket which got completely destroyed in air. Al Jazeera was actually one of few media outlets which correctly hypothesized that this rocket was unrelated to the incident.
The fact is, we still don’t know what happened, all we know is that many of the initial reports were false. There was a lot of lying involved to win the narrative (especially by Israeli officials), and there are at least two very credible hypotheses on what happened.
In this case, they have a good argument.