They are looking at several objects between approx. 1-20 Earth-masses (presumably they were bigger during primordial creation; this is their size, now, after 13 billion years of evaporation).
The bigger the black hole, the slower it evaporates. By the time you get up into planetary sizes they basically don't change mass on the time scale of stars. (From this wonderful calculator [0], a black hole with a mass of 6.0x10^24 kg - about that of the earth - would have a lifetime of ~5.75x10^50 years)
So if there's a 1-20 Earth-mass black hole out there, it hasn't evaporated from anything appreciably larger to get to that point.
Earth-mass black holes have surface temperatures well under that of the cosmic background radiation and will not have evaporated, they'd have been literally accreting mass just from the background. You need to get down to the billion-ish tons range to find black holes that can have appreciably evaporated over the life of the universe.
