Genuine question; why do you find it doubtful?
The scale of the destruction to the landscape is enormous:
Drone video shows striking images of Gaza before and after war
Indeed - it looks terrible.
However, again you can look at wars and see this kind of thing before. I think the Blitz on Britain would give some insight into this, and the bombing campaigns of the second world war.
Figures off of the top of my head as I don't like looking things up. I think the Blitz killed maybe 40,000 people in Britain. There were, as would be expected, many more injuries - maybe 100,000: possibly, both figures would be just slightly higher - approximately. ( As I said off the top of my head).
There were two million homes destroyed - very apt here.
At first glimpse, it looks like all those wonderful shelters and tube stations and brilliant organisation and spirit saved everyone.
Yet, Tube stations were locked shut and people cleared until the government just stopped enforcing it.
There were hardly any shelters. I don't think there were shelters for even five percent of people - until much later on. Obviously, priority people had shelters: The Admiralty/ Whitehall/ Parliament etc.
There was little in ground to air defences: I don't think that improved vastly until the V1s etc.
As for spirit, unfortunately (as one would expect) there was widespread looting and clearly crime. But I will accept the unquantifiable way that people could handle more hardship in the past. They were somewhat more stoic than now. (I watched everyone have a meltdown here over a power cut).
So even with two million buildings destroyed (crazy right?) there were 40 odd thousand dead across the country.
There are a variety of reasons for this. People tend to find shelter wherever they can - tube stations are an example of that. It was not a government idea - the opposite in fact. So where we see Gaza flattened - it does not demonstrate that everyone, or even a large percentage are dead.
As said, there are always miles more wounded than killed. If the Gaza figures are true then there are also well over 100,000 wounded - what would that look like? I can kind of tell you, that's people limping and laying down everywhere you look. That's people bleeding, people lining most streets on stretchers, blood streaming in the gutters. There aren't hospitals anywhere that can handle that. I haven't seen this - I've seen snippets, but what I'm talking about is more than what we're seeing.
Also today's weapons are more precise, although not quite as much as people think. If there's an Hamas guy in one flat - there will be collateral damage (to use such a blank phrase) but it will not be like carpet bombing a population for the sake of it. Israel are not just flying over Gaza dropping bombs everywhere. Otherwise the footage and photos would be, unfortunately, far worse than we're seeing.
People are largely in camps - and although people do get hit by stray munitions (or even targeted ones), Israel isn't just randomly napalming camps or anything like that. There are also obviously tunnels and basements. I could go on but I presume you can see what I mean.
Just informed opinions based on past experiences really. Obviously, I could be totally wrong.