Well, the forest service could have closed down that entire area, including the trailhead, if they so desired.
Yes they could. But it is easier to use boundaries that are already signed and to enforce closures where there are already gates. So when they say they have picked such boundaries, I believe them and look at a map. Sometimes you might need to look at a better map. They are available. They've been posted here.
Dale B. Dalrymple