puffling it's rare that I disagree with justabout or Stark
but actually, I think that focussing on the ASD thing, will do you no favours.
No matter how good your DD's vocabulary or diction, etc., are, she is obviously not using those skills effectively at school - agreed?
So, if you can step back from your worries that they are pushing her down an ASD route, then look at what the school are actually doing.
They are referring her to SALT, who have in their remit communication. If you don't tackle what seems at the very least to be extreme shyness in the classroom, you could end up in 'selective mutism' territory.
What the choice board does, is give your DD the opportunity to make a choice and communicate without the pressure of talking to adults who she either can't or won't talk to otherwise.
Frankly, my DD1 has a severe SAL delay, and talks the hind legs off a donkey, despite finding it hard to say what she wants to say at all. I would be much more concerned if I was in your situation, with my DD1 having the speech you describe and the vocabulary, yet unable to use it in school. I don't say that lightly.
I think it is time to focus on your DD here, and not your fears of stigma and labelling. Your DD is struggling. SENCOs have enough to do without making up problems. Children in schools struggle to get help even with an ASD diagnosis. Ask Genieinabottle. ASD dx with severe SAL delay, and very little help at all on School Action Plus.
If your DD doesn't need the strategies, she will ignore them and communicate her choices verbally. If she needs them, who cares what they were originally used for?
My DD1 is in a Special school. It doesn't matter why they are there, they all use Makaton, they all use visuals. In fact, I think it would be great if all schools taught all children Makaton, etc., much more inclusive.