PHP was practically "serverless" before serverless was even a thing. You just deploy your code with ftp, rsync, git, or whatever. There's no step 2.
"serverless" now means you don't run your own web server, you use someone else's.
PHP is very much comparable to serverless. All that talk about bootstrapping costs in serverless is something PHP developers have been tackling forever.
I believe the post you're responding to was alluding to the degree to which a lot of single-file PHP/HTML is written in a very "serverless-esque" way: no dependencies on an outer framework, and likely scales quite far compared to other languages when run on any number of servers (with the required libraries).