That doesn't mean it's not going to be difficult to use that parser. Not everyone has the luxury of being able to use third-party code, or having the time allotted to write a JSON5 parser. The JSON parser some places are using may have been written two decades ago and works well enough that there's little motivation to implement JSON5 support. Sometimes it's just company policy or internal politics that prevent the usage.
It's also just not that big a deal overall for the intended use of the DiffX format. It's mainly machine-generated and machine-consumed. There's human readability concerns for sure, but the format looks to be designed mainly for tools to create and consume, so missing a few features that JSON5 brings is not that big of a deal.
So the whole world should suffer through vanilla JSON because someone, somewhere, has an overbearing and paranoid software approval process? That's the attitude the delayed universal unicode adoption by a decade.
"That doesn't mean it's not going to be difficult to use that parser. Not everyone has the luxury of being able to use third-party code, or having the time allotted to write a JSON5 parser."
Why are these people the target market?
I understand it may be important to you, but that isn't the same as "matters to target market/audience".
On top of that, the same constraints you mention here would stop you parsing current git patch formats, and lots of other things anyway. So you were never going to be using modern tools that might care here.
This is all also really meta. Who exactly is writing software with >1% market share, needs to parse the patch format, and can't access a JSON parser.
Instead of this theoretical discussion, let's have a concrete one.