Beyond how that is contradictory, are you saying that the vendor is providing a non-mainline version of Linux that can't be updated because the driver API will be broken upon update?
Hardware-level stuff/embedded is outside of my area of expertise, but as I understand it you are pretty much correct.
You see this a lot with Android devices and custom images. There will be drivers that are only provided in the vendor blessed image, patches that are difficult or impossible to port to new versions of the kernel, etc.
Again, this is me looking in from outside. Most of my information has come from reading about other peoples experience with hardware, especially android devices but also other embedded chips.
Or am I misunderstanding you?