By categorizing all employees leaving as "discharge for cause, voluntary departure, or retirement", which are explicitly listed as 'not a layoff' in the WARN Act https://www.govtrack.us/congress/bills/100/s2527/text
I don't think they would be able to justify firing 12000 people for that, but sure, anything can happen under American law.
I can't imagine Meta winning in the end in California - the courts there are quite employee-friendly in such matters, and FAANG employees can generally afford lawyers.