I would like to see the condition of that Forest Service Road where they turned onto it. It might be in good condition there, with no hint it was a bad route, if a county commissioner lives a few blocks that way.

I have no idea what a "Forest Service Road" is. Never seen one; it's possible they hadn't either. Around here we have "Farm to Market" roads and "Ranch Road" designations, which are often major roads near town. I think that all of the major roads in Austin except the Interstate and one US highway also have an FM or RR designation. No particular reason for a city person from here to assume FM or RR means poorly maintained.

It's worth noting they seem to have been well prepared in every other respect, handling three days in those conditions apparently without difficulty, and may have been able to go longer.

Given that they did everything else right I'd hesitate to condemn the decision to turn onto the road with knowing what they saw when they took the GPS' advice.

PS. GPS errors can be more exciting than this. Errors at lakes formed by dams are surprising common. Old roads that existed before the lake & damn often still exist with the same name on either side of the lake, and GPS software generally assumes that if the map data in an area is blank that a road continues uninterrupted if it seems to be in the right place to connect and the name matches ... even if in reality both sides now end in a public boat ramp.

PPS. Another example of technology's helpful suggestions gone awry: I misspelled "condemn" above and my software's spell checker offered a correct spelling of "condom".