Strictly speaking, that's not true. Even if the submitter would still
be required to have copyright assignment for the FSF, they could be
copyable to the DMD front-end _as long as the submitter himself sends
them for inclusion there too_. This is the practical significance of
the license grantback from the FSF to the author.
This is getting off-topic for this list. Still, if this were the plan
(and I don't know whether it is or not), I think that the FSF would reject
it, because it would implicitly ask all GCC developers to help out with
a proprietary product.
There would also be a huge conflict-of-interest issue if the official
maintainer of the D front end were in a position to accept or reject
patches based not on their technical merit, but on whether the contributor
agrees to separately contribute them under the dual-license scheme, and
his/her employer had an interest in this issue.