He starts with our gravity-as-the-weakest-force conjecture (AMNV) and requires the light particles and their Compton wavelength to "fit" inside the black holes. This leads him to a new inequality written in the Planck units:
- M > 1/g
So if you have a very weakly-coupled gauge theory in your theory of quantum gravity, there are new light particles whose mass is
- m < g
and inversely, black holes only behave quite semiclassically if they're heavier not only than "1" (Planck mass) but they must also be heavier than "1/g". I tend to think that this conclusion could be correct although the "cataclysmic" language looks somewhat unfamiliar to me.
The small black holes, once they cease to be macroscopic black holes described by semiclassical gravity, become ordinary heavy excited particles, strings, or branes and one needs the microscopic description of quantum gravity to figure out any details. As far as string theory goes, this description is perfectly well-behaved and avoids any catastrophes.