Implementer here. I haven't set out to "implement an existing thing." Rather, the project is a combination of certain existing things that up to now haven't been available together in a single open source Prolog environment, SWI included. Some of these choices have far reaching implications in terms of what can/cannot be easily implemented -- see https://www.metalevel.at/prolog/attributedvariables. SWI is much more monolithic, slow moving and not particularly ISO compliant. I don't want to reproduce the entirety of SWI and its huge ecosystem. I want a trimmed down set of features that are core to what I want out of a Prolog system, which does not include 95% of SWI. I suppose it could, but I will leave that up to others, if that's what they want.

Originally, the project started because I wanted a Prolog that could talk to Common Lisp. Prolog powers the type inference engine of the Shen programming language, a Lisp with optional static type checking. Implementing Shen in CL was something of a no-brainer, because I knew I wasn't going to beat SBCL for Lisp performance. Plus, I wanted to learn the ins-and-outs of Prolog implementation and also retain some of my sanity during the Prolog-to-CL FFI part. That meant avoiding existing Prologs.