There's having Facebook and then there's using Facebook. I think most people who say they don't use Facebook still maintain an account for things that require it.
Facebook Messenger is like the ultimate opt in for contacts. People you don’t know at all get filtered when trying to contact you, but people you know tangentially or see once every few years are still reachable. I would have no reason to have this person’s number.
There are plenty of people that I might contact about something but I wouldn’t have their number.
I was also planning a surprise for my wife’s birthday and coordinating for her to have a girl’s night out with her friends. I wouldn’t have her friends numbers, but I was able to message them.