Glancing through the munitions in the article, HIMARS were originally developed and deployed in the 90s, not sure if they have a backup to GPS.

Thr Excalibur system mentioned does have an inertial guidance system to fall back on but I would assume it would lose some precision that GPS would afford.

Backdoor implies it was deliberately put there. Looking at the CVE it does not seem intentional so just an exploit.

“I’m worried about X.” “The fact you aren’t worried about Y is very telling. My friend is a Y-ologist and says Y is very serious.”