@rabbithole, when we speak of time-lock, we usually mean time-lock encryption. The underlying idea is that a message can be encrypted “to the future”, meaning it can be decrypted at a specific time (in the future), but not earlier than that.
As it is also described in the vetKD paper (in Section 1.3), time-lock encryption can be achieved with vetKD, for example, via identity-based encryption (IBE) by letting a canister smart contract (aka dapp) return an IBE decryption key corresponding to a particular time(-slot), only if the respective time(-slot) is reached (or in the past), and by letting some sender IBE-encrypt a message (or file, etc.) using the desired earliest decryption time(-slot) as identity (that is, as derivation ID).
Does that make sense?