It is splitting words. There is a settlement layer in lightning, which is presenting the preimage and unpeeling the onion HLTCs in reverse order. This happens at the latency of the network path, so usually less than a second. Bitcoin settling is usually tied to confirmation in the block, which lasts ~10minutes. Lightning might be IOUs, but ones that are fungible themselves and not tied to a specific debtor. Actual lightning-to-bitcoin cashout would probably not happen for everyday use, or at least not more often than you change bankaccounts in todays terms.