Here it's specified that for a scenario involving Exchange 2010 Hub Transport servers in the same AD site with 2013 Mailbox Servers,when an Exchange 2010 Hub Transport server transmits a message to an Exchange 2013 Mailbox server in the same Active Directory site, the Exchange 2010 Hub Transport server advertises support for shadow redundancy using the XSHADOW command, but the Mailbox server doesn't advertise support for shadow redundancy.This prevents the Exchange 2010 Hub Transport server from creating a shadow copy of the message on an Exchange 2013 Mailbox server.
Most likely this only refers to the Transport component of the 2013 Mailbox role (otherwise mails will be delivered via RPC from 2010 Hub Transport to Mailbox, right ? - or has this mechanism changed in 2013 ?). I don't understand the reason behind the 2013 Mailbox not advertising support for shadow redundancy, despite the second statement in the quoted text should give the reason.