I think you're confusing two things.
You need a tool to remove the doorbell from the mount. Some people might call it a "key," and
this example of a replacement does, but as you can see it's just a thin piece of metal. You can (and I have) use anything that fits in the hole to remove the camera. This is not a "key" in the traditional sense, and requires nothing particularly special. IMO it says absolutely nothing about any prior knowledge or access.
Second, a few people have posted completely unsubstantiated claims that the intruder may have had a key to the house. I personally find this unlikely, and I don't think we have seen
any credible reports or evidence to suggest it, but of course I am open to changing my mind if someone provides sources. I think this is just a random theory that someone posted in this thread and others ran with it.