At most it’s a description of an aspect of the project’s internal architecture. It means absolutely nothing to the end user.

I believe that the Nyxt website should have a big red banner warning people that, because it uses a rendering engine that is not conformant and, I suppose, is a work in progress, it can’t actually serve as a practical web browser. This would save people from wasting time installing it only to find out that it doesn’t work. And remove the misleading comments about it being engine-agnostic. I really don’t understand what they hope to accomplish by forcing people to discover the true nature of their project the hard way, but it leaves a bad taste in my mouth. The same goes for Suckless’ surf browser and others. For their part, the wekitGTK website falsely claims that their engine is “a full-featured port of the WebKit rendering engine”. As mentioned in another comment, these rendering errors do not appear in Safari, which uses WebKit.