It's just a form, there's a huge pathway for diagnosis for autism, and lots of professionals involved.
If it turns out your child doesn't have autism, happy days, hallelujah.
But if it turns out your child does have autism, the earlier it's recognised, and the earlier you're offered intervention then the less developmentally behind their peers they are likely to fall and their sensory needs will be understood and met - in theory.
I regards to speech and language, they should be working with nursery and offering them advice and techniques to use to help like intensive interaction with regular early help meetings so you can all discuss his progress, what works, what doesn't and what else needs to be introduced in terms of goals, whether there's a SEN pathway or not.
Regardless of autism diagnosis or not it sounds like they're concerned about some degree of global developmental delay, and they're right to approach you about this. If they did not do this, it would be neglect.
I don't think there's any reason to be mad at them for doing their job. And I don't think you've anything to lose by allowing them to start the pathway for diagnosis, as he will only be diagnosed if he actually meets all the criteria after assessment.