The answer lies in the sentence immediately before the one you quoted an excerpt of.
In a study published in December 2000, the star's diameter was measured with the Infrared Spatial Interferometer (ISI) at mid-infrared wavelengths producing a limb-darkened estimate of 55.2±0.5 mas—a figure entirely consistent with Michelson's findings eighty years earlier.[31][50] At the time of its publication, the estimated parallax from the Hipparcos mission was 7.63±1.64 mas, yielding an estimated radius for Betelgeuse of 3.6 AU.
(emphasis mine)
You combine the angular diameter with the distance obtained from the parallax to get the physical diameter.
$$\frac{r}{\mathrm{1\ AU}} = \frac{D_\theta}{2\varpi}$$
Where $r$ is the physical radius, $D_\theta$ is the angular diameter, and $\varpi$ is the parallax. The factor of 2 comes from conversion of diameter to radius.