The physics-y way to answer this question is usually to define things in terms of the scales at which it's probed. If everything you can sense with macroscopic

The physics-y way to answer this question is usually to define things in terms of the scales at which it's probed. If everything you can sense with macroscopic instruments is consistent with the liquid properties and the magnetic properties, it's a magnetic liquid. That it's "really" just solid magnets suspended in a liquid seems not much more relevant than the fact that solids are "really" fluids if you watch them on the astronomically long timescales on which they lose solid form due to tunneling. In this case, it's a liquid magnet down to roughly the scales of the nanoparticles, which is only a bit bigger than the atomic scale on which nothing is a normal liquid.