Why calling laid-off tech employees 'low performers' is career-ending

I doubt it. Most people won't pay attention to the stated reasons behind the multiple Meta layoffs. Of those who do pay attention many will forget about the details in a couple of weeks. Anyone who pays close attention and cares in the future will probably interpret the layoffs correctly as management failure. The only low performers in this story are Zuckerberg and his management team.

The CV prestige of a FAANG job comes from getting hired. Zuckerberg himself once pointed out that getting admitted to Harvard counted for a lot more than graduating (he failed to graduate).

Absolutely. Zuck is destroying 4000 careers.