I find nothing saying they didn't implement manifest V3. The only info I can find from them is that their own adblocker, which isn't an extension, will continue to work[0] and that, for example, ublock will continue to work as long as google doesn't remove manifest v2 support.[0] Seems odd to me that what Google does to manifest V2 matters at all to Brave, if Brave is indeed a fork and not just chromium with a skin.
0: https://twitter.com/brave/status/1574822799700541446
1: https://twitter.com/BrendanEich/status/1534893414579249152