I can explain to you the theory, but I can't vouch for the product. Most of the info can be found on that site you linked to though.
A RGB signal consists of R,G,B, Ground, and Sync wires. Scart is essentially the same, but the sync rate is different. The solution on that website is probably the best... take your composite out (yellow rca cable) and add a chip which essentially removes the color and luma info, halves the signal and essentially makes a sync line. So you've got 480i RGB coming out of your xbox, which should work nicely on a jamma cab.
The board you linked to looks quite complex, but it has to be. In addition to the sync chip it needs two chips to emulate xbox controllers, a video amp and an audio amp. It looks like the right stuff is in place, but the only way to tell if it's any count is to buy one.