Complete certainty of how it turned out is a bit too much to ask. For every story like mine, there's an equivalent story of a legacy system in boring Java/C that no one wants to touch, works like magic, etc.
FWIW, I wrote a pretty extensive documentation of the project before I left, including recommendations for improvement[1]. Again, it's foolish to claim complete certainty but the nature of the project was such that they are unlikely to need to rewrite anything on top of the engine. Any modifications and additions moving forward would be in the declared rules, not in any of the logic. And for that, they have hundreds of examples from me to copy from.
And if they do need to touch the "parser", at ~200LoC the complexity is certainly far below, say, a Spring J2EE code base. It's all in one file too, that shouldn't be too bad.
[1] If I may use HN as a small soapbox, actually, months later, I figured out a further small addition to that project that would've made working with it far easier. If any of my previous team recognizes this and this project is still in use, feel free to reach out to me for one last brilliant idea. :)