As far as I am aware, there is no proof. As you suggest, this is a
'fringe' hypothesis by a historian who is trying to construct an
alternative social model. As far as I am aware, there is no evidence
that he was anything but what he appears to be, an ordinary member of
the Iberian christian nobility. That could be because of the grand
conspiracy the Duchess suggests, to purge all mention of him being a
muslim, or it could be because he was just a member of the Iberian
christian nobility. Note that on the Wikipedia Talk page, a
contributor has adamantly insisted that he isn't muslim, he is jewish.
Basically, there are fringe historians who want to argue for a
greater minority role in history and will take anyone who doesn't have
a perfectly documented pedigree and speculate that it is because they
were 'other', where other is whatever flavor of minority they think
was more important (or want to descend from).
There are several things that have to be kept in mind with regard to
the evidence in this particular case. First, while we think of
Leon-Castile as a single entity, it was governed more like the United
States in that there were separate kingdoms within the Crown of
Castile, and they all had their own governmental administration. Thus
without seeing the context, we don't know that the 'export' may not
have been from the kingdom of Galicia to the kingdom of Toledo. To
speculate it was to Morocco is just begging the question. As to
'vassal', everyone in realm was a vassal of the king, not just moors.
This is an example of forming a hypothesis first and then looking for
the slightest evidence that could be interpreted in a manner
consistent with the hypothesis (evidence that could also be
interpreted in agreement with the standard formulation, and would be
if one didn't decide on the answer before evaluating the evidence).
taf