Both my children have autism. They get full time 1:1. The other parents know and they know why.
It simply makes things easier. Autism is nothing to hide. By guarding the information, it's like you're saying it's a secret. By saying it's a secret, you are implying there is something wrong/bad/shameful about it. It must be hidden. It's none of their business.
Well, no, it isn't any of their business, but it's not information you have to conceal either. If it helps your child have an easier time of it, isn't it worth it? What matters more? protecting his 'personal information', or making sure that he is accepted, included and understood? And yes, in an ideal world, everyone would be without explanation, but we don't live in that world. We live in a world where if you're different, people recoil
and are much better if they understand why you're different.
When my children joined a school, The staff would have a lesson on autism and explain it to the children. Explain the difficulties and the reasons why children with autism are different and different ways the other children could help them.
The other children were (generally, not always) great about it! Helpful, nice.
And then, of course, if you do run into a selfish parent who thinks that you are having a grand old time of it with a ta all to yourself you can go up to them and say "If you want my child's extra support for your child, I take it you want your child to have my child's autism too?"
Which normally shuts them up.