I had a look at the original source of the measurements quoted on the Wikipedia page (Zhang et al. 2021) and the problem is simply systematic errors in the models that are being used to infer the fundamental parameters. I.e, the masses and radii (and ages) of the brown dwarfs in that paper are systematically in error, which is a clear conclusion of the paper.
The paper is about modelling the infrared spectra and trying to determine the parameters of the brown dwarfs using a suite of theoretical models generated over a grid of surface gravities, effective temperatures and metallicities. The radii basically come from the "luminosity" of the spectrum combined with an effective temperature (Stefan's law) and the mass then arises from the surface gravity and radius.
The conclusion of the paper, which comes from considering the properties of a population of late T-dwarfs that were modelled in this way, is that the radii are systematically underestimated and that the masses are "unphysically" small - thanks to the underestimated radius and an underestimated gravity. I.e., they should not be used or quoted as the masses and radii of these objects. The problem is almost certainly the assumptions that go into the atmospheric models being used to fit the brown dwarf spectra.