No I didn't. I know how layoffs works in America because I've been laid off in America.

You know how it worked for you. What you described is not a legal requirement and many other people have had different experiences.

I think we’re all aware of how layoffs in the US work because there’s so godawfully many of them (that get publicised on HN). It’s very clear to me that 3-6 months of severance is very much not the norm. Being walked out immediately by security very much is.

Those two things are not mutually exclusive. Usually both happen. You usually get 3+ months of severance on your "don't sue us or badmouth us" paperwork and then security escorts you out.