Signing a letter does not equal encouraging anyone to do anything. Encouraging someone to do something is when you go into them for advise, they advise you to take this course of action. My RY is signed onto the “letter”, and (I have not spoken to all his talmidim, true) he does not encourage this course of action, at all.
When I call him to discuss potential shidduchim in the yeshiva for girls my wife knows, he does not look at age or anything the letter encourages. If he believes the appropriate match for this 20 yr old girl is a 25 yr old bachur, he says so, the same way if he felt a 20 yr old was the most suitable match.