Sadly we don't trust our security practices anywhere enough for that! Secret investigations happen in an air gapped room on computers with their network cards removed then get moved across to the main CMS when they're ready to publish.
Probably not, no, until they were about to be published. I imagine that the choice between "run an entire data centre ourselves, store everything there" and "use AWS, but keep high sensitivity stories on local machines" is an easy one.
After all, the client computer that connects to the CMS is just as, or more likely to be compromised. I wouldn't be surprised if the coverage (or at least parts of it) were edited on airgapped laptops.
> the choice between "run an entire data centre ourselves, store everything there"
If those were the only two choices, you might be right. But the resources needed for the actual CMS functionality sound modest enough to run independently of the main website.
> the client computer that connects to the CMS is just as, or more likely to be compromised