It is. The key part is that the computing is much more flexible in Xenos, making the use of all those special-purpose resources much more efficient. Also, the rops are apples vs oranges, because the xenos rops are much more powerful. Specifically, they can do 4xMSAA for free, and they can operate in Z-only mode, in which each rop can render 2 pixels worth of z (*4 MSAA).
For example, let's look at a simple forward renderer, with a 2:1 vertex/pixel workload split that's ideal for the X1800. Simple forward renderers, using early z out, have something like 3-4x overdraw. Let's be generous and call it 3x. This is more or less the best possible case for X1800XT.
However, because the Xenos has much more shading power than the X1800, and because it can use it flexibly, it doesn't have to render like that. Instead, let's use an otherwise normal forward rendering, but add an early Z pass in front. This adds roughly +100% vertex workload, and 50% to rop workload (before overdraw). However, we've just completely eliminated overdraw. Xenos now has some 2.4x the texturing power, and similar remaining rop power.
And this is something that MS taught everyone how to do when they shipped the dev kits. Once you move into deferred rendering and all the tricks it gives you, the difference only grows.
Source: I have shipped significant amount of GPU code on both XBOX 360 and PC. Comparing Xenos to X1800 XT and saying X1800 is more powerful by specs is like comparing a P4 to a Core 2 Duo and saying the P4 must be faster because it has higher clock speed. Xenos is much more efficient.
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