The word standardization gets abused a lot in our industry. Usually it implies something that 1) has a non ambiguous specification, 2) is widely used. 3) is endorsed by some standards body or cross industry organization or simply so widely used that it effectively is a standard 4) has multiple independent implementations (tools, parsers, websiters, etc.)
Org-mode has a specification (https://orgmode.org/); is at this point in time not widely used; has not been endorsed or proposed to a standards body like e.g. w3c, ietf, etc.; has a very limited amount of implementations. Declaring it a standard is meaningless until that is fixed.
The Github flavor of markdown has a decent specification (https://github.github.com/gfm/), many implementations of e.g. parsers for different languages, many websites/tools/etc. using it. It's not at this point a formal standard but the previous makes it more or less a de-facto industry standard that is widely used, with a reference OSS implementation and many other parsers aiming to be compatible.
If you are implementing markdown support, you could do worse than pick this flavor. Several other flavors are equally well specified, have similarly good tool support and industry endorsement, and might very well be considered standards in their own right.
Of course, like with Json, there's a long tail of not so great parsers that have implementation specific bugs/quirks/omissions/features, etc.