You never know how smart someone is until you give them a voice.
People assume that not talking means not intelligent. Kindly get that idea out of your mind. Besides being wrong, it is insulting. Frame your mind to consider that your assumptions about non-verbal autistics are wrong. Rethink possibilities for all autistics, giving humanity to all not just some that are called high functioning.
Point the way the way and help those who need assistance to things that will let them express their thoughts. When you can’t talk and have no way to communicate, you become eminent enwrapped in frustration, and longing to be heard. You wish someone would give you a chance to be heard. Sometimes it gets so bad you scream and cry.
If everyone assumed intelligence we wouldn’t have to go through that.