It's supposed to be editable, but not in the way you expect - TiddlyWikis are single-file HTMLs that can live on your desktop and are usually used for note taking. When you are finished with your edit, you can save a copy of the entire page using "File > Save Page As": https://en.wikipedia.org/wiki/TiddlyWiki

...unless this GH-userpage hosted version is special in this regard.

Got it, thanks! I will have to give TiddlyWiki a shot someday. It doesn't seem to fit my personal needs, but something the family can use easily.

With the node based tiddlywiki it saves the tiddlers as seperate text files