Only on HN would the NWS format be described as 'custom'.

Only in the US would you assume your govt's format is 'default'.

I think the implication was more that any government meteorological office's format is the default over a small private company.

Especially since this API is repackaging output from the NWS's GFS model output.

Yeah. And the follow up snark comment about it being an American thing. It is literally the file format specified for this type of data, globally. As outlined by the World Meteorological Organization (WMO) decades ago. It's widely supported and parseable. Making a web API that parses GRIB data is trivial. It's not some esoteric thing. You can open the files in python on any modern computer+OS.

So what is different or better in the Dark Sky syntax?

I know literally nothing about weather data standardization, but if the format was — for the sake of argument — unique to the US for example and another more popular format was used either internationally or de-facto by third parties, I’d say it qualifies as a “custom” format.

Quite the opposite. It's literally the global format that nearly everyone in the world provides data in.