I think someone doesn't know exactly what the difference between a planet and a moon is.
A moon (or technically satellite) orbits another object that isn't a primary object (ie: sun) and of sufficient mass that it at least has a roughly spherical shape, as opposed to a captured asteroid. It also can't be too massive, otherwise it would not be a moon, but a a partner planet (ie: binary or trinary).
Now let's go to google to find the official definition...
https://en.wikipedia.org/wiki/Natural_satellite
There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.[12]
The upper limit is also vague. Two orbiting bodies are sometimes described as a double body rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common[citation needed] dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass.
In other words... erm... its wibbly wobbly.