This looks like a very well researched paper that points out the malicious activity of a mining pool: F2pool.
It doesn’t break the technology since it is more a mining pool stealing the best transaction fees from other mining pool by lying on their difficulty level and replacing their block.
Pool A puts out a valid block -> Pool b puts out a valid block and lies on the difficulty level. The blockchain takes Pool B block and Pool A becomes an uncle block.
Pool B gets to choose the best fees for them as parent block due to their malicious activity.
