The most likely reason is that the folks who say that don't understand that the CVO RG Ultra is not lowered from the factory like the CVO RG Custom is. Your bike has the same rear tire size and rear shock size as the standard models, and if those lowering blocks will work on a standard model they will work on your bike. It's not the best way to lower a bike, as you already know, but it is definitely less expensive than buying a set of high priced 12" rear shocks. Might be worth trying just to make sure you really want to lower it, before springing for a set of premium brand 12" shocks. 
Jerry
What Jerry said...

May I also add that you can usually find a good, used set of the HD Premium or premium low shocks around this site and the interweb for a fraction of the original costs. The lows are 12" shocks and actually lower the bike about 3/4 of an inch. I removed the stock air shocks on my '15 CVORG and replaced with the premium lows for about $250 and saved labor at a dealer doing it myself. These were stock on my '12 SESG and I liked them for my '15. Others may point you to more expensive aftermarket but for your application, I'd go this route instead of considering the lowering blocks. I did the blocks once on my '95 Ultra just for grins and giggles...lasted about 2 weeks and took them off. I ended up replacing the air shocks with Fox on that bike.
If'n you decide to spend more in the future for upgrade or you don't like the premiums, you can always resell the premiums and get some of your money back. That would be tougher to do with the blocks
