Via drill size for heat dissipation
The larger the "thermally active" cross section of your via's plating is, the lower its thermal resistance will be. The plating's cross section is, with \$d\$ being the diameter and \$t\$ the plating thickness:

$$A \approx \pi d t$$

That means larger drill -> larger area; and thicker plating -> larger area. OTOH, some solder will get into the via hole and enhance heat transfer between the two board sides, but that's harder to quantify because the via is not necessarily filled with solder.

Given that the overall available area for thermal vias is usually fixed, I'd use a larger number of smaller vias in order to fill a larger fraction of that area. 0.3 mm sounds reasonable.

Can you show us numbers of other via diameters, or did they just make a statement about 0.33 mm?

A thermal simulation would show more detailed insight, but that's probably a bit over the top.