That "crater" is awfully small and far away from the damage. Something landed there but to my untrained eyes it supports the idea of an off target missile. The article also says it's not consistent with a typical bomb drop appearance.
Hamas rockets have killed plenty of people in the past. They aren't shooting off bottle rockets. They have hit people in Gaza many times before with failed rockets.
Do you remember the last published statistic on that? IIRC from previous massive Hamas rocket barrages, somewhere in the range of 10-30% of them never make it to Israel, but instead end up landing inside Gaza.
What are the yields, usually? AFAIK, Hamas' rockets are your pretty standard rocket artillery designed for use against relatively small fortifications, personnel, or armor.
If Israel was basing their assessment purely on the aerial imagery they provided and didn't have access to (or didn't trust as authoritative) the pictures from the ground I can't understand why they came to that conclusion. That little pockmark crater is visible in the top view but it's not clearly identifiable as a crater until you see it close up. They were probably looking for the type of huge, impossible-to-miss craters that get created by their own JDAMs.