This is kind of true for desktop and maybe high end laptops but here we're talking about a chip for ultrabooks. Those laptop designs don't have a lot of headroom for cooling and power delivery. The listed spec is accurate.
Note that it is possible to basically overclock the chip and raise this limit up to around 50W for the combo GPU/CPU if you really want to. This requires enhancing the cooler as shown in [0] and no OEM does that out of the box.