The bigger a piece of glass at the same thickness the easier it is to break. Thats why really big windows are not paper thin. You would need to increase the thickness to keep the same structural integrity at some point. That's why OLED TV's bend easily if they are mishandled. Thin metal and with very thin glass is a fragile design but TV's don't get handled every day. Google bent LG OLEDS to see what I mean. Basically the new iPad is following that same design ethos, but for a tablet you need more rigidity, and if the glass seems to deform with the pencil that shows a weakness straight away. I was really wanting one of these but now I'm tempted to wait till next year for a few tweaks in the strength department.
I don't have to google anything to know what kind of effect glass has on a casing, I'm more than aware of its properties.
If you take a bare metal iPad case, no glass on it, from any generation, it is going to be relatively easy to bend.
If you add a piece of glass to that equation, it becomes harder to bend for a few reasons, but let's just keep it simple.
The main two we're concerning ourselves with here, is that it's now a complete box, with no missing surfaces and the rigidity of the glass. This combination shores up the casing, making it inherently less flexible and also distributes any forces applied to it across more surface. The case protects the glass and the glass strengthens the case, its symbiotic.
If that glass get broken, the structural integrity disappears completely. hence why we see such a difference between the first video posted and the second. The first video's bending is being done on an already structurally compromised iPad, in the second video it's not.
Using your logic you should love the new 12.9" iPad Pro, it's smaller than the previous generation. So, using your logic, smaller glass = less easy to break. The size and indeed the thickness of glass is not the only measure of its strength or rigidity. Try a piece of iPad sized glass from the 1970's against a piece from today's iPad, you'll see what I mean.
Is it possible to bend an iPad, even without breaking the glass? Yep, definitely, but only a small amount, there's enough give in the glass to allow for that. Is it possible to bend and break an iPad, sure, with enough force. Is it likely, well I suppose that depends on how haphazardly you treat your devices.