PKCS11: CKA_VALUE_LEN attribute and Unwrap key
Yes, I think that's sufficiently unclear to ask for clarification of the authors of the document, and it probably needs fixing.
However, what I do know is that the key size is definitely clear from the wrapped key itself. So there is definitely no need to specify CKA_VALUE_LEN during the call to C_UnwrapKey. There is no need to check it either; I'd be more worried about the value of the key then the size of the key if the key can be altered somehow.
What is possibly meant is that the key size of the wrapped key is determined using CKA_VALUE_LEN during key generation. Because in that case the type by itself doesn't specify the output length, i.e. the size of the key being unwrapped.