[Tool Agnostic]

The User Manual and the manufacturer's response in the Home Depot Q&A for your Ridgid R4518 table saw both say that you should use a 6 inch dado stack. They say that you should NOT use an adjustable (aka "wobble") dado on it. Steve Ramsey somehow missed the details and made a simple error, that's all.

The R4518 is a jobsite table saw. A 6 inch dado stack has less mass, and it puts less torque and stress on the motor and bearings compared with an 8 inch dado stack, so the recommendation makes sense to me.

Many woodworkers prefer to use a 6 inch dado stack even though their larger saws could use an 8 inch dado stack for the reasons above. The primary drawback of a 6 inch versus an 8 inch dado stack is that you lose 1 inch of dado stack height. There are very few projects where that actually matters. Some people argue that the fewer teeth on a 6 inch dado stack yields slightly rougher cuts than an 8 inch dado stack, but I doubt you would notice the difference in practice.

Bottom line:
Buy a six inch dado stack for your Ridgid R4518 jobsite table saw. It will do everything you want, and you will be very pleased with it. Steve probably missed the fine print in the manual.