NEC ECC: Do limitations of liability in X18 extend to capping liability to Delay Damages?

It would depend upon the wording in your contract. Start by looking at whether Secondary Options X7 and X18 have been amended from the standard text and then see what Contract Data Part 1 states about these options, which should clarify the situation.

A common approach is capping liability for delay damages at, say, 15% of the total of the Prices at the contract date and to 100% of the total of the Prices at the contract date, for aggregate liability.

No, they are explicitly excluded in clause X18.4, second bullet.