That's not true. You can bivouac (== 1 night stay) anywhere in France where it's not explicitly forbidden (e.g cities/towns/national parks etc). I did a small part of it last year and slept in a tent 5 out 6 nights.
Yes, according to the creator, 70% of the trail has bivouacs as a possibility. So ⅔ of the trail. With 1 town every 3 days of hiking, it should be totally workable (though perhaps expensive to stay in towns a lot).
It'd be about 20 miles per day if you're just on a normal visit with no visa at all. Nothing too out of the ordinary for people who do this kind of long distance backpacking.
Camping appears to be allowed along most of it according to the website.
If you're doing low profile onenighters there's about zero chance to be bothered about it. Source : I did it a long time ago, I crossed France with my backpack from east to west then went in England.
Only once a woman asked me what I was doing in her field, I asked politely if I could stay the night, she said one night is OK and that was it.
The only reason I had this encounter is because I was so tired that I didn't bother to look for a hidden place. I just built my tent in the middle of her flat field.
If people are considering doing this either camp after sunset and be gone before sunrise or knock on the farmhouse and ask if you can camp in the corner of a field in their land. 99% of people will say yes and appreciate you asking (you might get some free food out of it too!), and the 1% who do care will be pissed if they find you which you want to avoid anyway.
If there is one country with many campings its france. You find them even in the smallest villages. Wild camping is not really necessary. Camping muncipals are very cheap.