How reliable/accurate is Apple Store's Diagnostics for iPhones? Anyone

Hey fellow Ifixiter’s- some context to the situation.. been iPhone battery issues which I planned to replace DIY and just being lazy. On a drive down from San Jose in the Central Valley the temps went up to a crisp 105 degrees F.. I tried to power down/off electronics but stupid phone needs to be tethered to the charger in case need it or ward off death by boredom. Yet the A/C was broken in my car..

Though also worked fine till I got to LA (of course) toward the end of the drive.. Temps were in 90’s.. suddenly phone died and wouldn’t hold a charge or reboot. I hoped it was just a battery issue— maybe discharged too low beyond its capacity. Apple Store says Logic Board fried.. Based on scenario, could def be true.. Though didn’t have much confidence in him or his solution (just buy a new 700 dollar iPhone! Hmm Noo-Kay!) Hope errors can be made and diagnostics can be inaccurate? Any experiences or feedback or vote of confidence in Apple Store “Geniuses” is gratefully received. Thanks for your time and consideration.