The press isn't beta testing the single player, they are playing a beta with the first chapter of the single player campaign in hope that they will write / post about the game, thus giving it more exposure, which in turn should hopefully result in more sales.
Whether single player or multi-player, as RTS is a significant part of the game, the units need to be balanced and the AI refined. The most effective way to do that is with multi-player. That is what the beta is focusing on first, to get the most feedback on.
There are actually previews / videos coming out, such as the (20 minute)
Let's Try Divinity: Dragon Commander - Single Player Campaign.
