To be fair, BBC's sole reason for investing in Dirac (to my understanding) was to fit HD quality video into the pipes for an SD production system. As far as I know, it satisfied that purpose for them, so the project can be considered a success from that standpoint.
The BBC does have skin in the game. They have many people in house who have invested in HEVC, and put their names behind that decision; furthermore, maybe you'll remember when the BBC spent a huge amount of money on Dirac, which seems to have continued to be extremely impractical in the medium to long term. Maybe the BBC doesn't have a spectacular track record for picking the winners in the video codec game. ;- )
Added: The BBC's comparison also seems to be between professionally-configured and tuned HEVC/VVC encoders, supported by their vendors; and what seems to be a default-configured libaom, with no consultation with the vendor.