::UPDATE::
I learned two things today.
1) I confirmed with the Goal Zero tech support that while the Yeti 500x is designed to be charged from an external USB-C PD source, that source MUST support higher than 5V for the negotiated voltage (either 15V or 20V). If you have a USB-C PD power source that only supports 5V output, that will NOT charge the Yeti 500x. It just sits there and does nothing after it fails to negotiate. No error message. No indication that it even tried. Just nothing. This is both undocumented and impossible to troubleshoot. Bad job Goal Zero!
2) I may have been way too quick to bash their $40 car charger. I think this thing is significantly smarter than I gave it credit for.
Since the USBC charger thing didn't work, I ended up buying their 12V car charger adapter anyway (needed something as a plan-c last minute solution). I plugged it into the cigarette lighter port on my car and turned on the car's aux key position so the port would go live but didn't turn the engine on because I didn't want to waste gas. The LED on the charger came on so I know the 12V was live, but it refused to charge the yeti 500x. Again, nothing. No error, no indicator, nothing. I confirmed the AC-DC power brick charged from the wall and was about to take it back as defective, when I had the idea to turn the engine on first.
As soon as I turned the engine on, the LED color on the 12V charging cable changed from red to green and it started charging the battery bank! There appears to be at least one additional component in this box that can tell if the engine is running, and only pulls current in that case. This is actually really smart since you don't want to accidently drain someone's car battery just to charge a utility battery. The plastic case has no screws and appears to be glued together really well so no taking it apart without mangling it. Sorry.
So there you go. Sorry, Goal Zero. Your $40 boost converter + possible engine detect circuit in a box is probably an acceptable value.