Well, if the mempool clears from a mined block it will be almost empty for the next minute or so.

Meaning, the agreement could have been "only mine this block if the total fees that you would get from a normal block is less than 100k sats"

It would take longer but with the current fee market it would have been mined eventually (mempool frequently empty recently) for a cost of $20. Or take 10x on the numbers then $200 but the expected time to mine it is divided by 10.