I don’t think anyone’s intention was to make it seem less chamur than it is. All they are saying is that they are working on it at their own pace with or without your help. Your unsolicited advice just makes it that much more difficult.
If the mussar came from someone they respected, they would likely take the lesson and attempt to apply it. However to someone they feel does not come with such pure motivation, they may justify themselves.
Regarding lashon hara, the Chofetz Chaim was known to have been a shmoozer. The reason people were actually willing to learn his sefer was because they knew it was a struggle he definitely understood!
For mussar coming from a woman (or man) who cannot fathom the difficulty it is for some, is a slap in the face to someone who makes the choice of the length of their skirt every minute of the day. Yes, I know you have an obligation to the Torah to protect its values regarding tzniut. But is that really what its about? Or does your rebuke come from jealousy, hate or disrespect for the other person. Pinchas obviously did have pure intentions, but if your intentions are from anything at all other than a sincere love for the mitva, you’re probably better off keeping quiet.