How do they get such accuracy?

When one reads various post here and on other sites about 3d printer design there are many articles about making the moving parts as light as possible.
This often translates to making sure the steppers are not on the moving axis and using a bowden extruder to keep the weight off the moving parts
Makerbot Replicator 2 claims to have superior accuracy compared to the many other designs out there, please note that I have never seen one apart from in photos so
I might be wrong about this next bit but.... They seem to have broken all the 'rules?? They have a complete extruder on the moving axis and that same axis also seems to carry the motor that moves it.
The Y axis appears to be driven by 2 belts. I have read many articles where people blame belts for poor accuracy of print.
So what does this all mean? Are their claims exaggerated or are these design issues not really a cause for concern?
I would really like to hear some thoughts about this as I am currently in the process of designing a 3d printer and would like to make an informed decision regarding this.

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They seem to have broken all the 'rules??

They didn't.

To get good accuracy you "just" have to avoid the printer bending, vibrating or doing other unwanted movements. This can be achieved either be slowing down movements, especially acceleration, or by making the frame stiffer. Replicator 2 comes with a welded steel frame, so it's pretty stiff and allows decent acceleration at good accuracy.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

MBI has also been known to be a bit, let's say "optimistic" about their advertised speeds, resolutions, etc. Most people here err the other direction.

Also, if you have ever seen a Makerbot print, even with accelerated FW they wiggle like crazy because of how heavy the print head is.