Looking at the website, it seems like it's limited to MD, without support for MDX - which is a shame, as MDX has become a powerful tool in how we write documentation of late -- especially since their output strategy is "click to host this as a website" -- perfect for MDX.

Also, it seems even more odd that their markdown support is a custom flavour - adding support for things like tabs, and warning styles. (Unless I've missed something?)

It's unhelpful for a WYSIWYG tool to show you something of how it's rendered in a very small subset of the markdown ecosystem.