No amount of testing will guarantee perfection in a program. The real problem here is that Microsoft effectively allowed CrowdStrike to hack into the core of their operating system in the name of security. Maybe, they should pay less attention to AI nonsense and more attention to making their own operating system secure without giving it over to a third-party.
Lots of people speculating that this was due to a lack of testing. Philosophically true: in theory, *even more* testing can find all possible defects. But we can assume they *did* test. Si probsbly something subtle went wrong.