“Anonymous” is a super broad term in tracking. All it means is that you are given a unique ID by Facebook while they track you. But that ID is the same across any site that integrates Pixel. So they have a metric ton of your browsing data tied to the same ID even though it’s across a few dozen or hundred websites. They also use that same ID when you’re on Facebook itself, so they can serve ads based on that ID instead of your Facebook account. It helps them to skirt some privacy requirements while still building a super detailed profile of you.