My understanding (and IANAL and Jorge is welcome to correct me) is
that the IETF
does indeed have "sufficient rights to allow re-use of IETF
documents within the IETF", and
that this is purely concerned with the power of granting
modification rights to other parties.
This is not a very common occurrence as far as I can tell, and so in
this is a corner case.
You are correct that the rights for the IETF Standards Process are
already in place, at least for every contribution made after RFC 2026
was published. However, RFC 5378 does not include a provision for a
contribution that does not grant all of the required rights.
Even if the IETF Trust were to never make use of any rights beyond
the IETF Standards Process, these additional rights must be granted
under the requirements of RFC 5378. If a person cannot obtain the
necessary rights, then that person cannot make a contribution to the
IETF. This was the consensus of the IPR WG and the IETF, and the
IETF is now operating under the resulting process BCP.
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