"Thinnest device" means that the object being referred to has the least thickness or is the thinnest among all other devices of its kind. Full definition
It is not a flat design, but the slimmer proportions give you the feel of holding a very thin device. (indianexpress.com)
Certainly, there'd be no huge increase in battery size supported as a result, or a dramatically thinner device, either of which would be an easy pitch. (slashgear.com)
It is tough and rigid, the display is very well protected, and I'm especially impressed by the total absence of keyboard flex from such an exceedingly thin device. (theverge.com)