I mean, they know they're stealingg it from a real person, and no matter how careful you are, they're tracking you.
Even if you don't have a profile, they have a "you shaped hole" and someone somewhere has tagged you in a picture. They still have data on you built from people in any picture you were tagged in, their data gets extrapolated to you.
The ID is so they can sell the data for a higher price because it's "verified" to be yours. Even tho without it they still 100% know it's you.
This isn't about Facebook getting your ID, it's just so the people they'll selling it to pay more.
There's a chance a human never looked at OPs Shrek picture, an AI may have just checked it against real name tagged photos and it didn't match. Even tho OP doesn't have a Facebook account under their real name.
Facebook still knows what OP looks like, and almost assuredly knew the burner account was theirs