That may be true, but the current Lexus pricing model is for either an Intermediate or a Full service, so with a Full service you may or may not get new spark plugs depending on years / mileage, but there is no difference in price so I would see no point in declining the spark plug change.
For example my next service due is 60k miles / 6-years but I will only be at around 25k miles, so unless I downgrade from a Full to an Intermediate service (which would probably negate Relax) there would be no point in declining a Spark Plug change (unless there is some specific option not listed in the pricing). At 8-years I will pay the same price for a Full service but won't get new plugs.