@PickAChew really nails it: "This reflects exactly how much the actual SEN system lets down parents who aren't excellent communicators (or, as can so often be the case, have some additional needs of their own.) so much of the time, it relies on parents being able to advocate effectively and sometimes aggressively for their children just to get their basic legal right to a "suitable" education met."
This is so true and a grim reflection of systemic, ablist practices which actively disadvantage families and create, rather than remove, barriers to children and young people accessing education.
I really feel for you, OP. I'm a teacher and SEND parent and have supported pupils' families to make direct applications for ECHP needs assessments as the process schools go through is clunky and, let's face it, even the most clued-up, empathetic SENDCO in the world won't match a parent's commitment to getting their child's needs met. Local authority resources are gatekept through a cynical process of attrition and running down the clock which leads to poor outcomes for children.
For all the posters suggesting that schools do not off-roll or make mendacious referrals; this absolutely happens. The definition of off-rolling includes school leaders deliberately making a child's experience of school, or the family-school relationship, hostile or strained to the point where parents agree to their child being taken off a school's roll or request this as a result of thinly veiled coercion.
OP, my advice for you would be to lean in to the referral, be open and transparent ‐hell, ask them how they may be able to support you and make it clear you welcome their involvement. They can help with so many things. I'd take care not to be defensive, confrontational or apportioning blame; just emphasis your efforts at working with school and what it is you would like to happen.
Good luck to you and your DD. This is becoming a distressingly common reality for many of our autistic girls.