@instagibbs is talking about HTLC-Success and HTLC-Timeout transactions as defined in BOLT3
These are presigned spends of HTLCs made with SIGHASH_SINGLE|SIGHASH_ANYPREVOUT that allow multiple of them to be combined into a single transaction. The don’t contribute any fee (or any meaningful fee), so we would normally expect them to be combined with an extra contributed input that adds fees (and likely an extra output for change). However, the threat-to-decentralization argument, you could pay a miner out-of-band fees to mine them without adding an extra input or output, saving block space.
I made this same argument to PT last night (last paragraph): https://twitter.com/hrdng/status/1743293516992876561