I don't know if the XML is better than JSON thing still holds with this year's frontier models, but it was definitely a thing last year. Here's Anthropic's documentation about that: https://docs.claude.com/en/docs/build-with-claude/prompt-eng...

Note that they don't actually suggest that the XML needs to be VALID!

My guess was that JSON requires more characters to be escaped than XML-ish syntax does, plus matching opening and closing tags makes it a little easier for the LLM not to lose track of which string corresponds to which key.