I could add an answer after this question got locked.
It seems lock is broken
Lock prevents the question from being edited. We use it to stop edit/roll-back wars, among other things.
Now, you also cannot add answers if a question is locked.
For info; if we want to stop you adding answers, we close it then lock it; keeping the two concepts separate allows for a lot more granularity.
Just a note -- we did change the behavior of lock, when applied at the question level, so that it prevents new answers from being added.