You need to educate yourself. Seriously.
If you think ALL people with hidden disabilities have no physical effects so don't need a BB space, you need to think again.
JHS, CFS, PoTS, all forms of EDS, and many other condiions can be invisible, a person can walk into a store seemingly okay, but after a very short distance may need support.
For example, my DD can sometimes walk in a confident manner to the store, but then leans on the trolley and finds that an okay compromise for her despite being NHS wheelchair assessed and offered many years ago.
She only goes out on a day when her mobility and other disabillities aren't affecting her too badly. You may see her walk "confidently" into a store from a BB space and wonder, but you've not seen her on the previous up to 20 days at a time before that not having the ability to make the trip there.
If you followed her round the store on one of her 'good' days, you'd see she needs frequent rests and often leaves me with the trolley while she sits on a bench in the store until she can carry on.
Her BB renewal came at the time we were waiting for her PIP Tribunal As it happened, the BB assessor scored my DD's mobility lower in the 'how many yards' category than the PIP assessor had done.
She was granted a renewed BB before the Tribunal sat and the BB asessor's examination was presented as more evidence at Tribunal.
There's no such diagnosis as "very mild autistic" again, education would help you. Also autism is rarely present alone, it usually has one or several co-morbid conditions which alone would pass the BB criteria.
I'm sure you are very knowledgeable about your own childrens' disabilities, but you have no right or currently knowledge to judge anyone else's BB provision based on observing them for a minute or so from getting out of their car and walking to a store.
You have absolutely no idea about their medical conditions or capabilities, nor should you.