They tell you that only the latest branch is secure because as far as I know there is no official policy for supporting old versions. That means that support for old branches is not guaranteed. In practice however, you can see that they are still providing patches for version 3.7, which was originally released in 2013, and got last patched a week ago in version 3.7.33.
So while in theory you should upgrade to branch 5.4 as soon as it comes out, in practice you can keep your 5.3 version as long as you install the patches (5.3.1, 5.3.2, etc.), which should get installed automatically by default anyway (some kind of internal cron provided by WordPress). The risk of WordPress actually dropping support for a recent branch is almost non-existent, in my opinion. Older branches are more at risk of course.