I wonder about this too. They say they delete the template (the statistical model they know is your face). Ok.

So now I register a new fb profile, using a picture of you, opting in to facial recognition. Now what?

They can't match the picture against your profile, because they've deleted the template?

This is actually an exception that GDPR makes for storing data - companies can keep around some sort of record to "tombstone" users.

Sort of. GDPR is so woefully under-specified in some places, and general in others, it's really entirely up to the will of the regulators what would count as a violation.