> So, a factor of 14.4 is enough to put the line between Ceres and Pluto, but a factor of 23.0 is not enough to put it between Pluto and Mercury?

Why not? It's an arbitrary line, and there has to be a lower limit somewhere.

In the IAU definition, hydrostatic equilibrium loosely defines a lower limit for all planets including dwarf planets, but at the limits that's a fuzzy property that gets debated for small bodies. In fact Ceres is less massive than what some studies have found to be required for hydrostatic equilibrium, e.g.: https://iopscience.iop.org/article/10.3847/PSJ/ad55f3 . The abstract of that paper starts out with, "The current IAU definition of 'planet' is problematic because it is vague and excludes exoplanets."

If we had more small planetary candidates in the solar system, they would have had to come up with some additional constraint to separate planets from non-planets.