In the "Emergent Magnetic Charge" section, second paragraph:

"Finally, we emphasize that the observation of emergent monopoles is fully consistent with the modelling of AFM topological textures in Fig. 2 and does not violate Maxwell’s equation as they are, in fact, sinks and sources of the H^ field."

So this is a collective effect of a bunch of magnetic dipoles, and they go on to describe how div B is indeed zero in the material.