Wrong. The WARN act does not differentiate between mass firings and mass layoffs of full-time workers. Requirements change once certain firing thresholds are crossed. Firing enough people to cross those thresholds, telling them individually that they are being fired for performance reasons, and not giving advanced notice would be a clear violation.
https://www.dol.gov/sites/dolgov/files/ETA/Layoff/pdfs/Worke...