  Two very similar proposals have been made recently for witnessing
nonclassical features of gravity, by Bose et al. and by Marletto and Vedral.
However, while these proposals are asserted to be very general, they are in
fact based on a very strong claim: that quantum systems cannot become entangled
via a classical intermediary. We point out that the support provided for this
claim is only applicable to a very limited class of quantum-classical
interaction models, corresponding to Koopman-type dynamics. We show that the
claim is also valid for mean-field models, but that it is contradicted by
explicit counterexamples based on the configuration-ensemble model. Thus,
neither proposal provides a definitive test of nonclassical gravity.
