Like you say it's not surprising.
Remember there's no internet or telecomms in Gaza that's not Israeli. You can't google anything or text anyone without the Israeli Intelligence agencies seeing it.
So for an attack to happen on this scale - 22 simultaneous attacks, Air, Sea and Land, massive hostage taking - this has been planned for probably over a year, and you can't do that without using electronic communicaiton which means it slipped through the net probably hundreds of times.
Then there's the HUMINT side of things...people on the ground, informers etc. These attacks involved hundreds if not thousands of people in HAMAS. How could they not have any sources?
IIRC The USA recently warned Israel that the situation in Gaza and the West Bank was getting out of control and combined with their own constitutional crisis and upheaval they were risking something big happening.
I suppose there's only 2 outcomes. 1) People rally behind the flag and just allow the right wing takeover that's been resisted in Israel for the last few years. 2) People blame Netenyahu's corruption and attempts to make himself a quasi-dictator for the attack and he's booted out as soon as the dust settles.
WHich one of those happens probably depends a lot on the outcome of the hostage situation, which could take years to sort out.