I think he means that the UK govt has mandated a particular CSV standard for tabular data, at the same time. I think you're right that OpenAPI has nothing to do with CSV.
Yes. CSV with the obvious extra conditions "UTF-8" and "one header row" is fairly interoperable, but note: "when you’re using Excel for Windows to load a file that is UTF-8 without a BOM, you can resolve the display of non-ASCII characters by using the ‘import text’ option". If a Windows user just clicks on the file then sometimes UTF-8 is interpreted as some 8-bit encoding and you get mojibake. Nevertheless, you shouldn't add a BOM because the standard doesn't permit that and it might cause all sorts of problems on other systems.
Is that true? Iirc Openapi lets you define schemata for csv payloads in POST body sections. Whether they are validated by the backends is another question...
The page title is actually "Improve CSVs and API descriptions with these Open Standards Board recommendations"