France has a rate of 69.30 children per 10,000 compared to 78.10 for the UK. Source: https://worldpopulationreview.com/country-rankings/autism-rates-by-country
However, I would strongly suspect that the lower rate of diagnosis is at least partly due to the fact that autism and neurodivergence is still such a stigma in France.
My inlaws moved to France and we could have gone with them, but life would have been spectacularly shit for my DC (both officially diagnosed). Autism and associated spectrum disorders really are considered to be "less than" in French society. In 2018 - which I know was a few years ago now - United Nations said that France was 50 years behind with its treatment of autism, and also said that autistic children were being subjected to "widespread violations of their rights".
Autistic people are far more likely to be institutionalised, and be separated from the community. Autistic children are not widely welcomed in mainstream schools. The barbaric process of "le packing" was outlawed in France in 2012 but they've continued to do "studies" on the effectiveness of this "treatment". (Packing is basically stripping a child to their underwear, wrapping them in freezing cold, wet blankets and then talking at them while they're cold and shivering for 45 minutes).
I can't imagine that any country which has such shambolic practices towards their autistic community would be taking their autistic and neurodivergent children out to public spaces very readily. You're only seeing the ones deemed to be acceptable.
Of course, there are multiple factors also at play - and I think it's a combination of lots of the answers given by PP. Socialisation of children, attitude to meals, fewer screens, stricter parenting, etc. But please don't underestimate the impact that your child's ND brain will have on their behaviour - it really is very hard when you have ADHD. It's not a made-up condition as many strict parents often assume and you can't discipline the neurodivergence out of your child.
While no one wants to see children screaming at the table and running around, I'm not especially keen on seeing rows of quiet, obedient children who are overly subservient to adults either. Going off to play between courses seems perfectly reasonable to me.
Also, I'm autistic and ADHD too. So according to France I'm worth less as a person. France can get to fuck.