Yes, the tapes are compatible given the limitation of the DataFort tape header. According Fabric OS Encryption Administrators Gude v6.3.0 (53-1001341-02 August 7, 2009 published by Brocade):
Only DF version 2.x- and 3.x-compatible NetApp DataFort (DF) tape metaheaders and block formats
are supported for reading, decrypting, and decompressing the tapes.
Only DF version 2.x- and 3.x-compatible tape block formats and metaheaders are supported for
writing and encrypting tapes in DF-compatible format.
A DF-compatible license is required.
So the limitation is that the compatible DataFort tape metaheaders is a 2.x or 3.x revision. The 2.x and 3.x are the release levels that most DataFort tape encryption customers have used over the past 4 years.