It wasn't a virus definitions update. It was a driver update. The driver is used to identify and block threats incoming from wifi and wired internet.
The "Outage" section of the Wikipedia article goes into more detail: https://en.wikipedia.org/wiki/2024_CrowdStrike_incident#Outage
Their computers may not be affected, but their everyday lives might be. Some of the affected services include 911, stoplights, banks, hospitals, and a whole other smorgasbord of stuff.