No one's calling into question that there was an explosion. The question is over the cause and the extent of casualties, which people who lived next door likely wouldn't be in the best position to evaluate.
I think too many people don't realize how hard it is to really estimate these things even if they happen right next to you, and especially if they happen at night.
It's hard enough to estimate the caliber and distance of a single pistol or rifle shot (with training you can get pretty good), let alone the difference between a smaller mortar and a larger JDAM.
It doesn't take a genius to tell the difference between a hospital building being flattened and a fire in the parking lot. The people actually there knowingly lied, end of story.
A fire in a parking lot + courtyard where there are many, many people encamped because it's a safe refuge in an active warzone? Even if the building is still standing, that could still mean hundreds dead.
Even if you don't imply known malice, you have a major factor - if a bystander is asked a question they very rarely will say "I don't know" or actually just describe what they saw. It's a well-known effect, and why analyzing recordings can be much more effective.
It's hard enough to estimate the caliber and distance of a single pistol or rifle shot (with training you can get pretty good), let alone the difference between a smaller mortar and a larger JDAM.