Again: I'd trust a standard netbook with standard Linux distribution more,
especially when kept offline. If
USB cables can be used to
access your computer, how long will it takes before "offline" devices come with their own hidden internet connection?
It's not even needed to have an old notebook. An USB stick with Tails OS and a few reboots actually can do all you need for pretty much all the safety you need and an easy to use cold storage.
But some do prefer the convenience of HW, some do prefer to use it together with their smartphone, ... and this kind of setup may be OK for them.
And those devices can have their own "hidden" internet only if they get connected through wires (which you should clearly not do) or you enter there your WiFi password (which would be even more stupid to do).