Obviously not telescopic observation, but they didn’t detect any other similar size objects in the same orbit, and I doubt you will find anyone seriously considering that an object several times the size of Earth could have comparable mass in its orbit otherwise. Just like nobody would say they aren’t round despite not having seen them.
Actually, something I like about the orbit clearing definition is that it would definitely apply to most of the extrasolar planets we have found, since some techniques like redshift more or less require them to not be part of belts. There are a few systems where the transit method has found a few large objects in the same orbit, but as dichotomies go, I think that one holds up better than almost anything else.
Of course, the IAU technically requires planets to go around the sun, so these are all exoplanets instead. But aside from that.
