My guess on that is this: the gps directed them onto the forest road well before that gate. They may never have seen the sign. My Rand McNally mapbook shows a unpaved road coming off of Hwy 89 north of Jacobs Lake and intersecting 67 down near the park entry station. I just looked at 40 or 50 maps online and none of them show the unpaved roads.
I know the park has such a map because we were using it and wanted to go to the Tuweep overlook. Not being a fool I checked with the ranger at Jacobs Lake USFS station and was warned away since that road was reported impassable even for 4WD vehicles. We went on the house rock trail instead.