"possibly" here means that while Y is hosting derived work, even the uploader doesn't know that for a fact since the software doing the uploading doesn't expose

"possibly" here means that while Y is hosting derived work, even the uploader doesn't know that for a fact since the software doing the uploading doesn't expose this detail. Knowledge of how to combine them being a derived work is an interesting theory. One challenge is that facts aren't typically copyrightable. On the other hand you could argue contributory infringement instead.