The comparison with being gay, and schools telling parents is so confused.
The guidance isn't saying parents have to be told if a child mentions they are trans. No, parents have to be consulted before school take any action which would support this. Or parents have to be told if this is relevant to the childs well being or other safeguarding concerns such as mental distress.
In the same way schools don't have to tell parents if a child says they are gay. But they would have to tell them if they had knwoledge that a child was on Grinder , or if the child was experiencing distress linked to their sexulaity.
In both cases they'd make a safeguarding deciosn about how safe it was to share the information to parents and if concerned involve advice from outside agencies.
Schools do not have to reveal every incidental pice of inmormation they have about a child. They are not the gestapo. But they do have to share every piece of information related to the childs well being and decsion making to parents.
It is supporting transition that schools cannot do without parental consent not an obligation to 'out' children.
e.g. if a child says they watch anime yu dpn't ahev to tell parents.
If the child says they are watching 18+ anime you need to riase this.
If there are concerns about a childs mental helath you have to share that the information about anime may be relevant.
The guidance is applying the same safeguarding rules that apply to evrything else including being gay. Trans should never have been viewed as a special case. That's safeguarding 101.