I assume you are right too, JSON is a less verbose format which allows you to express any structure you can express in XML, and should be as easy for AI to parse. Although that probably depends on the training data too.
I recently asked AI why .md files are so prevalent with agentic AI and the answer is ... because .md files also express structure, like headers and lists.
Again, depends on what the AI has been trained on.
I would go with JSON, or some version of it which would also allow comments.
But I'd be happy to hear about studies that show evidence for XML being more readable, than JSON.