Download the maps so you can use them offline. Have a backup battery so you can charge your phone if it dies. Keep your phone in a waterproof case or ziplock bag. Either be in a group with multiple phones (what's the chance that in a 3 person group everyone loses or breaks their phone?) or carry a paper map backup. This guy broke at least 2 of those rules.
The points the guide made that convinced me were: 1. You're already carrying the phone, it's silly not to utilize it. 2. Phone gps is so much easier to use than a map that you're much likelier to check it on a regular basis and therefore notice something is wrong earlier than if you are relying solely on a paper map. 3. Paper maps are great in some cases but they really suck on low visibility or dense trees. There are techniques for navigating in those situations but they aren't very precise.
Now, you do still have to have some level of competence and preparation. The trail this guy did was on altrails. I would've studied the trail there. I would've studied the route on caltopo. I would've consulted a paper map or guide book. I would've looked at the area on Google maps. I would've made sure everything seemed consistent between those different sources (if he had done that, he wouldn't have ended up on the wrong trail). But ultimately, I would've used caltopo to generate a PDF map of the area, loaded that map into avenza maps, and used that as my primary means of navigation.