The Xbox 360 devkits had extra USB ports for emulating DVD drives and high-speed data transfer. These kits required a fair amount of labor to put together; they were $10,000, and often hard to obtain.
I believe the Xbox One devkits are just production units with different key material (which prevents commercial titles from running, and allows developer-signed bits to run). Theoretically any Xbox One is a devkit, with the right software provisioning.
Dev kits usually have extra memory for debugging overhead and occasionally hardware for cycle specific profiling and the like.
Most release games run up to 5mb(or less) to the system memory limits so you can't turn on debugging and waiting for LTCG builds which can take 30mins+.
Modern game systems are running on top of a real big boy kernel this generation, so you don't need hardware level debugging support. An equivalent to gdbserver is enough.
The switch has a USB-C port though, which would be more than enough bandwidth for debugging. Mobile dev's have lived with USB 2.0 connections for building games just fine.
The problem is likely just that it becomes easier to install third party software if there is some kind of Dev mode inside retail units...but Nintendo's locked down hardware gets jailbroken anyways so I don't see the point in hiding it.
