I was thinking along the same lines as you, this just seems to be something that someone stumbled across, as if Apple are getting it ready to release as Beta, but aren't quite there yet despite the banner/links on beta.icloud.com. For example, the online instructions page that's mentioned a couple of posts above this wasn't there when this story was first published, the link just took you to a generic iCloud mail page with no reference to custom domains. I would have expected those to be in place before the beta was officially launched.
The inconsistent and unpredictable behaviour of this right now (I get different results every time I try exactly the same thing) means that Apple are doing a lot of final configuration and tweaking behind the scenes and we'll see it properly go beta within the next couple of days. Fundamentally, a beta is meant to be a "we think we've nailed it, we just want to check it in the real world" and not "this is very much a work in progress".