Aha, so now we get a "how was that done?" debate. Here's my entry...
It looks to me like there's gaps between the leg tops and the surrounding slab. If that's true, the legs aren't really connected to the slab, but rather they just stick through it. I think the base is a separate subassembly, and the slab just sits on it. (Okay, with a few screws to keep it from wandering around.) I think there are beams running across the slab from leg to leg. Probably mortise and tenon from the ends of the beams into the legs. Then there's a beam running down the midline of the slab connecting the crossbeams together.