>>> Why? A locked topic does not accept further posts irrespective of any moderation status.
Not a 100% correct statement; from regular users, yes it doesn't, but it still accepts messages from admins just fine. The difference is that if also moderated, the message goes into pending, but if not it sails through.
>>> a topic can be Moderated or it can be Locked, but it cannot be both at the same time.
Wait, how can you say that when one can set the topic to both? Try it and you'll see, moderate a topic and moderated icon shows up. Then lock it, moderated icon disappears and gets replaced by the locked icon. Topic is now both locked and moderated but only the locked icon shows up. Unlock it, locked icon goes away and gets replaced by moderated icon.
This works differently than the sticky icon which "appends" to any existing constraint/icon.
>>> I would argue that having both Locked and Moderated flags would be a source of confusion.
I personally don't see how it would, when the system itself allows one to set it to both. If nothing else, to me it's a visual reminder that I have set that topic to both constraints.