I had to look at the paper itself to figure this out, but - in this paper, they designate the binary (star + stellar remnant) together as M51-ULS-1, and the planet actually orbits the two members together (a so-called circumbinary planet [0]). The logic of the nomenclature system actually breaks down in such cases - here's a quote from the Wikipedia article on naming conventions [1] stating as much:

'According to Hessman et al., the implicit system for exoplanet names "utterly failed with the discovery of circumbinary planets", and they state that it is unhelpful. '

So, yeah, take that for what you will.

[0] https://en.wikipedia.org/wiki/Circumbinary_planet

[1] https://en.wikipedia.org/wiki/Exoplanet_naming_convention#Ci...