Even today, web formats cannot render documents with the same fidelity as ODF. Especially not for spreadsheets.
Remember, many people feel that neither ODF or OOXML are sufficient and reach for things like LaTeX.
I think LibreOffice also supporting Markdown is a nice nod at simpler open formats.
Perhaps you have not dug into the problem deeply enough.
ODF is certainly better than OOXML at any rate.
> Even today, web formats cannot render documents with the same fidelity as ODF. Especially not for spreadsheets.
That's a nonsense claim—literally a category error. You're conflating the limitations of what can be represented in a given file format with an implementation's (lack of) support for doing things encoded in that format.
LibreOffice doesn't render documents encoded in plain-text, browser-compatible formats with the same fidelity as ODF because LibreOffice has not implemented encoding documents in plain-text, browser-compatible formats that it renders with the same fidelity as ODF.
There is nothing in ODF that's inherently unrepresentable in JSON or pure XML.