In the US, the creator of content is automatically assumed to be the owner of a copyright, even if it is not officially filed with the Copyright Office. In fact

In the US, the creator of content is automatically assumed to be the owner of a copyright, even if it is not officially filed with the Copyright Office. In fact, all that official filing gives you is the right to receive treble damages. But not filing doesn't mean you give up your copyright. That being said, it looks like the content owner themselves gave up their right. This pretty much sums it up: "I release my copyright on this content." I'm not sure if that statement without being signed will carry much weight if the content creator later changes their mind and then attests that they didn't write the above statement. What would be safest would be for them to assign their copyright interest to you, or to sign a document that waives their copyright interest. IANAL, so if I'm wrong, you're entitled to a refund :)