Yes, you did miss something.
The tooltip for “Enable for Base Image” clearly states, that this only works for non offical images:
Trigger a build in this repository whenever the base image is updated on Docker Hub. Only works for non-official images.
Base images are specified in the FROM: directive in the Dockerfile of the source repository.
Which makes perfect sense, as an update of offical images would trigger a rebuild of all images, that use it as a base image. Would you prefer a denial of service because the build servers are overloaded or a blockage of the the build queue for a long time?