Call me a curmudgeon, but I just use an editor and markdown for the same fluid outlining. I guess this is more marketed towards people who don't also fluidly use a markup/markdown syntax.
This nice think outliners can do that editors generally can't do is Focus In / Out. Maybe not important for your thought process, but for me I really like being about to start new on a fresh "page", while still being in the same document.
Code editors can do it. It should be possible to define a syntax that would work well with this (no keywords, unquoted strings, : and TAB to define scopes).
