The two people you are talking to are intimately familiar with web protocols, how they work, and how to build and work with these systems in the real world
AP punts significant things to JSON-LD, ergo, if you want to be compliant with anything using those things you need JSON-LD. The "it's just JSON" line is almost pure marketing
Also we're discussing how these systems get used in practice. For instance, how to parse what is returned by https://xoxo.zone/users/KevinMarks