I can absolutely assure you that a rear motor car with less weight on the front wheels does not have more front traction due to less weight. I think there is a bit of a misinterpretation being made from that book as it is a bit confusing to explain clearly in less than a book page.

For a clue, look at the wording under the Figure 1-1 in that book. This is where it gets confusing. Pay attention to the middle section of the paragraph.

"As weight is increased, the traction also increases." Now lets look at the following sentence. "The important thing that must be recognized however, is that the increase in traction becomes less and less as weight is increased." They sound contradictory but they aren't.

Take a look at the extreme left side of that graph. With zero weight, we have zero traction. That seems logical. If the tire can't touch the ground it certainly can't grip it in any direction. As the graph progresses to the right, notice that as weight goes up, traction still goes up. As the second sentence I am quoting states, at a certain point, in the case of this graph around the 1000 lb vertical loading point, each additional pound of vertical weight does not give a full pound of lateral grip although it is increasing. Below that point each pound in vertical weight increase gave more than a pound of lateral traction. That was the tipping point.

RC cars are very light. So light in fact that they are effectively always on the left side of this curve. This is why adding weight increases traction to the point that it can increase total cornering ability at a higher g level. As with everything it is only to a point as this graph shows since too much weight will start to hurt performance. Weight has other side effects though with a big one being that it is more kinetic energy to dissipate in a wreck which results in more broken parts.

The thing to realize about weight distribution is that more weight forwards results in less yaw torque leverage that can be applied to the chassis from the wheels that are steering. This assumes of course that the front wheels are what are steering. Weight shifted rearwards allows more yaw torque leverage (please don't nitpick over that chosen term!) to be applied to the chassis by the wheels that are steering. As easier way to state this is more weight forwards equals more tendency to understeer and more weight rearwards equals more tendency to oversteer. Some people add weight up front and actually gain steering though. Ignoring the other things affected by a weight shift such as spring rate balance and relative roll stiffness changes, part of this is due to the fact that we are on the left half of that graph and that the added weight just gives the tires more traction. It is all about balance and not going too far though.