I don't get how this is supposed to work.

It looks like someone with a minimum share of the hashing power can enforce a minimum fee, up to the point where the total fee paid by legitimate transactions willing to accept that minimum drops off. The miner enforcing the minimum fee can flood the network with transactions paying just below that fee, and get back a sufficient portion of their own transaction fees that the fees they pay in spam transactions mined by others are balanced by the fees they earn from legitimate transactions.

I fail to see how this results in that miner achieving a monopoly. The other miners will be enjoying the benefits of the higher minimum fee, and will collect some of the fees from spam transactions, without having to pay for any spam transactions of their own. So whatever profit the attacking miner is making, the other miners will be making more.

At best, this attack allows a sizable minority group of miners to engage in price fixing without running out of money, under the constraint that legitimate transactions are still wiling to pay enough to fill half the block.