The mitigation is referred to as "defensive fee-rebroadcasting" and is discussed in subsection 3.4 of the paper. The sender mentions that when there is a backlog in the mempool and the defensive fractional fee HTLC-timeout gets stuck, it gives an advantage to the attacker. They also suggest that an attacker could replace-cycle multiple honest HTLC-timeouts with a single malicious HTLC-preimage, paying the absolute fee but only incurring the RBF penalty. The sender admits to not testing this specific behavior, but notes that the "fees" math doesn't appear to be in favor of the defenders. The email concludes with a farewell.