You would normally just keep using the original device tree. If the carrier board changes, or if a new non-plug-n-play device is added, then you might need to change something in the device tree. If the new kernel adds a driver, and the driver is for hardware needing the device tree to be modified, then you would change this. However, most people with a custom kernel won’t ever need to change the device tree.
Yes, the 4.x docs are showing how to build it all in one doc, but the 5.x doc is talking only about the kernel. The device tree part in the 4.x doc, although valid and useful, mentions a step not needed in many cases. It’s just thorough.