It’s more similar to Linktree - basically a link you put in your social media profile, and then from there you link to other things. A bit like boook.link (but that’s only for books)
About.me is a similar tool, but they're not open-source. This means you can't self-host your page for free if you wish, fork & make changes to the codebase if you want to add a new feature, or verify that the code handles your information how you would expect.
To us, this means about.me is a tool you can build with, whereas Singlelink is a platform you can build on top of.
Let me know if I can clarify any more, I hope this helps! :)