Doing a quick compare with the 12.1 Release notes - https://www.freebsd.org/releases/12.1R/relnotes/ - I will speculate that the 13.1 Release Notes are not a finished document, and quite a bit more will be added before the April 26th (current schedule) "final" Release of 13.1.
From the original post, "Please note, the release notes page is not yet complete, and will be
updated on an ongoing basis as the 13.1-RELEASE cycle progresses."