When one is using it on themselves like you describe it is a form of reclamation. Same goes with trap.
Though I disagree with you and find "trap" to be uber offensive even as someone who is also a femboy, because it implies I pretend to be a woman to trick people, really gross.
You are spot on that these words are mainly offensive when used against people pejoratively, in that context they absolutely are slurs.
Who told you or implied you weren't? Under the Egg Prime Directive that is not allowed.
Seriously, it's really not okay to go around telling other people that they are or aren't something personal and self-defined as gender identity.