That link was new too me, thanks! However: I wrote some chess-program myself (nothing big, hobby level) and I would not call it hard to implement. Just harder than what someone might assume initially. But in the end, it is one of the simpler simulations/algorithms I did. It is just the state of the board, the state of the game (how many turns, castle rights, past positions for the repetition rule, ...) and picking one rule set if one really wants to be exact.

(thinking about which rule set is correct would not be meaningful in my opinion - chess is a social construct, with only parts of it being well defined. I would not bother about the rest, at least not when implementing it)

By the way: I read "Computationally it's trivial" as more along the lines of "it has been done before, it is efficient to compute, one just has to do it" versus "this is new territory, one needs to come up with how to wire up the LLM output with an SMT solver, and we do not even know if/how it will work."