It's important to note that this is not the same thing as HBO Now, which will be available exclusively though Apple TV for three months. (That also costs $15 per month.) While you won't need an HBO subscription to watch the channel on Sling TV either, this definitely isn't an offer to get HBO all by itself.
In other words, this is still a bundle. But even at $35-$40 per month, it is undeniably cheaper than cable.