Submission title is editorialised, there is no mention of "solving" Rummikub on the page. That claim was why I clicked the link because I would've been surprised to see a Rummikub "solver". To me, a solver implies perfect play. This is a bot that uses heuristics to evaluate a move comparable to a competent human player, which is not the same thing as perfect play.

Interesting anyway, but I imagine this approach is similar to how many card playing AI works on things like Hoyle Card Games.