What the article doesn't make clear (but hints at in the last paragraph) is that the bomb didn't explode. Hence it was made safe & removed. So bomb fragments were not found all over the building and there was not an explosion consistant with an aircraft bomb. Would you really know what sort of bomb it was if it had exploded?
26-year-old sustains serious injuries in Tel Aviv stabbing (Haaretz)
from the article: Report: UN evidence counters Israel's account of Gaza war