However, as it should, it has you select it based on purpose, not based on kind number or whether it supports markdown. If you want to write an article or blog, you select "Long-Form Article", which is kind 30023, and it will have markdown support. But if you want to write a short note that shows up in the main feed on the majority of clients, then you can select the Short Text Note, and that will not support markdown in most clients. You will just have to hope people view the note in one of the clients that renders markdown for kind 1.
There is no rule against rendering markdown in kind 1, of course, and most clients render SOME things like images, videos, and link previews, instead of leaving them as plain text. It's just not expected by the protocol that they do so.