I saw that letter and I have mixed feelings about it.
First, the father loving his step-kids more. These kids are kind of young--it could be that they are misinterpreting the actions of a live-in father, taking the step-kids places, buying them things, doing things with them, as his loving the step-kids more, when it is just a fact that he spends more time with his step-kids, so he knows more about their day to day activities and sports and hobbies and favorite TV shows and latest fads. This could be interpreted as "loving them more." So, to me, this one thing could be true or it could be the kids' perception and not completely true. Not enough info in the letter to say for sure.
The bit about him loving them more than their mother? Manipulation. Not wanting to pay child support? Manipulation.
Most of the divorced parents who have custody that I've known have bent over backwards to present a good picture of the non-resident parent. The kids have all figured out on their own, as they got older, exactly what both parents were really like. But the custodial parents didn't lie about the other parent. If he/she screwed up, the kids knew that. The custodial parents would try to cushion the blow, but more because they didn't want to see their kids hurting than that they were protecting the reputation of the other parent. It's a fine line between protecting your kid and letting them experience the unvarnished truth, in some cases.