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.

“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.”