Yes, and furthermore, Infineon's 300mm GaN wafer technology is...

Yes, and furthermore, Infineon's 300mm GaN wafer technology is growing GaN on a substrate (there are several processes for doing this, using a variety of different substrates including Si and SiC). BLG's devices are made using wafers cut from a GaN boule. I believe the leading edge is still 150mm for growing GaN boules - 200mm at most. And the commercial sweet spot is probably still down at either 100mm or 50mm.

Here is a link to a recent paper on the subject of growing GaN boules:

https://pubs.rsc.org/en/content/articlehtml/2021/ce/d0ce01572e

I've only ever seen 50mm wafers in BLG literature, but I'm sure they also have the capability to do 100mm epitaxy, as indicated by this pic:



All IMO.