Not at all. Vendors just fork the kernel at the fixed releases Android versions ship (ex, 3.4, 3.10, 3.18, etc) and then merge in all their proprietary bullshi

Not at all. Vendors just fork the kernel at the fixed releases Android versions ship (ex, 3.4, 3.10, 3.18, etc) and then merge in all their proprietary bullshit violating the GPL nonsense into that tree, ship (and publish) that kernel source tree, but never merge back into Linus' tree. As a result, the device is supported only by the kernel they provide, rather than by generic Linux. And their kernel never gets updates.