The UN is hardly friendly to Israel, and I can hardly blame Israelis for being reluctant to talk to them after decades of one-sided resolutions and censure. If the UN finds “clear and convincing information,” you can bet that it’s real. They would take any excuse to say (as they have uncounted times in the past) that Israel is lying.
Israel, and the UN, are telling the truth (in this instance).
Again you do your cause no good by whitewashing Hamas. Their murderous brutality does not make the Palestinian cause any less (or any more) just.