Children can get funny ideas about finance.
If you'd asked me at that age which child in the class was richest, I'd have said Christopher.
He got for Christmas his own TV, video player and satellite dish, and £300. I was seriously impressed. We still had a black and white TV. They must have been so rich! I thought getting a book token for £3 was riches.
Looking back, if he did get those, which I doubt, they were probably either stolen or bought on hire purchase and went back after Christmas, and I'm fairly certain he was being at least physically abused.
In our class we had a child who I now know was incredibly rich, but they still came to school in their sisters hand-me-downs, and didn't have anything obviously expensive.
There were also a couple of the class who had riding lessons. One owned her own pony. Which again to an adult could mark them out as on the rich side, but as a child I saw as no more a symbol of money than having a hamster.
My dd thought we were about to run out of money one day when she'd asked for a magazine she often got and I said "Sorry, I don't have the money for that." What I actually meant was genuinely I hadn't any cash on me at that point of time. She told her friends that we were very poor which was a bit embarrassing. 
So it could easily be a comment from your ds, which he's probably forgotten about now.
It doesn't sound like the Chessington nor playdate situation is anything to do with money. Some parents are happy to host playdates at home. Others aren't. I was saying to dh that we'd have struggled to do playdates now if the children were that age because he's been working from home since Covid and the only space is downstairs which is fairly open plan.
I also hated having them sprung on me. I did occasionally, but only with parents that I didn't think would judge the tidiness of our house!