They used to have (custodial) LN as the trading currency. When that was enabled, you could list a SELL offer where bank transfer was the payment method, for example. So a trade for that involved no on-chain bitcoin transactions.
HodlHodl has not re-implemented that, they simply added LN as a payment method.
The hurdle for HodlHodl is that there's (currently) no easy way to escrow the LN without doing it custodial, and HodlHodl does not want to be a custodial platform whatsoever.