Exactly. They can suggest whatever browsers they want. Much like how Google suggests Chrome whenever any page with their logo loads. It's not like Facebook is saying if you aren't using on of these three browsers then we can't help you.
The page is STATIC. It isn't being generated based on any logic.
So the "you're" bit is vacuous. They don't mean your browser. They don't [functionally] mean any web browser. The page makes a statement, but we need to interpret that statement IRRESPECTIVE of our considerations of how it is generated/produced.
Needless, ridiculous, digressionist pedantry. The point was (and you can verify this yourself) that the page reports visually identical content regardless of which browser hits it. So it's not telling you that "your" browser is unsupported, it's a redirect target for some other page.
