Are Hashed Timelock Contracts and Zero-Knowledge Contingent Payments the same thing? This answer to How is a node in the middle prohibited from keeping the money in a routed Payment in Lightning network? describes how a Hashed Timelock Contract would be used to exchange the "secret" for a payment. This sounds a lot like the Zero-Knowledge Contingent Payment described by Greg Maxwell.

Would I be correct in assuming that the HTLC would be implemented including a ZKCP?
Not really. Both use OP_CLTV, in order to recover from a situation where one side refuses to participate in the protocol, but other than that, they're pretty different.

ZKCP has a zero knowledge proof communicated outside of the blockchain. On the other hand, HTLC passes signed transactions back and forth to update a channel.