It's ridiculous advice like this that will hopefully raise flags within schools.
No safeguarding lead with any credibility would think that it is appropriate to involve siblings in a secret that needs to be kept in school away from their parents. Even if they have already somehow managed to accept the idea that keeping the secret in the first place was in the child's best interests.
Hopefully, any such safeguarding leads are surrounded by colleagues who can challenge this. The most obvious challenge being to cite the impact of social transition from the Interim Report of the Cass Review. The more widely this report is circulated in education, the greater the understanding of the harms associated with the affirmation pathway. I hope the guidance references it and breaks it down in a clear way.
Teachers who want to support social transition need to become outliers. I suspect that the majority will be doing so out of a misplaced understanding of how best to support children but there will be some activists in the mix too. Any that continue along the secret affirmation pathway once there is clear unambiguous guidance (I'm not holding my breath on this current draft, given the potential gaping loophole in the "exceptional circumstances") should be managed as a safeguarding risk. Even if they are doing it out of kindness and support, they are a risk to children.