The rules don't say, "You may not politely tell people how they can play more effectively." They say we're not allowed to "compel a playing style".
"The intended use of Veraero and Verthunder is that you take advantage of the Dualcast trait to cast them instantly as the second spell after a faster spell like Jolt. They're not meant to be slow-casted on their own, as you would only be able to cast 2/3 as many spells this way." is a reasonable explanation of basic RDMing and no reasonable person should find this offensive.
"(Insert Skill Name) (Please use it.)" is usually a reasonable request that does not fall under any of the bullet points of the Offensive Expression section, except possibly the catch-all final bullet point, but a reasonable GM should not support the classification of this type of request as "offensive".
"If you don't use (Insert Skill Name) we're going to kick you" is quite different from either of the other examples and obviously an example of the sort of imperative that the play style rule is meant to guard against.