I looked up "header bidding" [1][2] and basically it seems to be a way for sellers to auction their space to all potential buyers at once, rather than letting Google be the middle-man and rig the game in its favor... for example, by reserving the last bid in an auction for Google Ad Exchange.
No wonder Google is against it.
[1] https://www.lotame.com/back-basics-header-bidding/
[2] https://adage.com/article/digital/how-header-bidding-wars-le...