BOLT #3: Shouldn't timeout be included in the script of "Offered HTLC Outputs" for the local node?
Pierre answered my question on the lightning-dev list and there is a pull request in order to make it more explicit. So the answer is that although the timeout is not addressed in the script, it is enforced in the HTLC-timeout transaction with an absolute timelock in the nLocktime. When the local node tries to spend the HTLC output, it has to explicitly set the nLocktime to cltv_expiry which prevents it from broadcasting the second-stage HTLC-timeout transaction until that time has reached.