No, legally you do not have to give credit for the free or purchased version of this assets. Taking a look at the license which is nothing more than a simple .txt that says hey you cant and can do this is not a legitimate license. The creator never invokes the CC 2.0 license clause nor states whether he is licensing it under CC BY,ND, or etc. Thus according to CC and Digital IP licensing laws makes the assets fall under a free open license and can not be changed since it has already be released to the public and would be illegal to revoke.