Well, in a sense a presupposition, but not without some basis.
"Our God is in the heavens; he does whatever he pleases" (Ps.115:3).
But I don't think the point is just speculation about what God _might_
do. Belief in "the contingent rationality of the universe" (Torrance)
has to do with God's work in creation. The belief that God has created
a rational universe is, I think, connected with the _goodness_ of
creation. _Inter alia_, it means that creation can be understood by
rational creatures. But this rationality is _contingent_ - i.e., God
could have created universe which were rational in other ways. The
possibility of that from a purely mathematical standpoint was first
shown with the development of non-Euclidean geometries. Theologically,
contingency is indeed related to divine freedom and the belief that God
creates _ex nihilo_ in the fullest sense. Scientifically, it means that
we have to observe the world in order to find out how it really is.
Pure reason, to which possible but non-existent universes are
accessible, cannot decide _which_ possible universe is real.
There are also presumably a lot of universes which would not
allow the development of intelligent life. (The whole AP argument comes
in here.) If God's freely determined purpose in creation is ultimately
connected with Incarnation, then those universes would not be suitable.