The 4850 is equal, if not faster than the 3870X2. The 3870X2 is about 1.5-1.7 times faster than a single 3870, so the 3870->4850 was about 1.6 times. The 4870 is faster yet than the 4850, so the 3870->4870 jump was greater than 1.6 times, more like 1.8-2.0 times as fast.
This article is comparing RV870 to RV770 as well, not RV790. If a 4890 is about 1.2 times as fast as RV770, and overclocked it is about 1.3-1.4 times as fast, the RV870 will only be 20-40% faster than a 4890.
If RV870 truly has twice as many shaders as RV770/790, this doesn't sound right at all, unless the clock speeds are a lot lower or something. So I hope something got lost in translation, or that this article is plain wrong.