Microsoft seems to have decided to entirely break this expectation such that closing the lid of a laptop puts it into this weird not-really-suspended state, such that putting it in a confined space could cause it to overheat. It's... pretty user-unfriendly and IMO stupid.
Ultimately the company selling to the end user has to stand over the package they're selling.
Closing the lid does not necessarily turn the laptop off at all (by design). I think most people have had that experience. In fact the lid may be closed with the laptop fully on.
In any case, they make is explicit and clear exactly to avoid any such assumption.
The title is obviously clickbait because of course you can put your Dell laptop in a bag and of course that does not void the warranty.
Weird that you're angrier about a headline than a warranty being voided by putting a laptop in a bag.
Or in other words, from the perspective of the hardware, what is about to happen once somebody closes the lid?