Not true. Often managers are caught unawares of layoffs until they are executed.

i knew someone who was QA his boss approved his PTO to go to the dentist. however that day turned out to be the layoff day so HR threated him to come to the office. was hilarious. god i hate HR. anyways if you ever go on PTO turn your phone off completely.

Threatened him with what? Being laid off?

I assume at the point he didn't know, so yeah, that could have worked (and was probably basically it).