The point that everybody here is trying to convey is that the radius of the object as given in one of the earlier posts is there for a particular reason other than observations, and as such comes burdened with certain assumptions.
The observations gave only the mass estimate.
Now, to get the radius, we have to make an informed guess as to what is the object made of. There are a few options:
1. It's made of normal matter (like planets and stars).
2. It's made of degenerate matter (like a white dwarf).
3. It's made of neutronium (like a neutron star).
4. It's a black hole.
Each of these options is in principle viable, in the sense that one can imagine objects made out of those materials. Each gives a different radius estimate for the same (observed) mass with identical gravitational effects.
But since we're making an informed guess, we can use our knowledge to eliminate some options. In particular, we can ask:
1) whether there is a way for either of the objects to form while having such low mass
2) whether such objects should produce additional observables that aren't there
...which eliminates all but the normal matter object, i.e. a planet. Plugging in a range of possible densities of normal matter (depending on possible compositions and using models for gravitational compression), you get a range of radii.
So, if you then take that radius and try to plug it into an equation for calculating a black hole mass, you're making two mistakes:
- you're ignoring the fact that the mass is what is already known and can't be anything else
- you're using the radius that was a guess based on the idea that the object is definitely not a black hole
