If you click on the link saying "Allura's features" on the front page you're taken here: https://forge-allura.apache.org/p/allura/wiki/Features/ which has some screenshots.

Can't you click "Wiki" and see it in action? Beats a screenshot.

No, it doesn't beat a screenshot. Their purpose is totally different.

Screenshots (on a landing page) usually depict a view the creators think is important, and beautiful to show the visitors.

A wiki to a screenshot is like a man page to... A screenshot.

After having made a look at the tool though I understand why there is no screenshot. It is basically an uglier and slower version of gogs.

I think he means that the wiki is part of Allura, so it's more like a demo. You can try it directly, see here: https://forge-allura.apache.org/p/allura/git/ci/master/tree/