Unless I missed something, though, the highest bound the author reported for the relaxation was 271 2/3 moves, which is obviously significantly higher than 218...
> With this improved model, I tried again and after ~23 000 seconds, Gurobi solved it to optimality!
Ah, I was not aware that that's what this language indicated. Thanks for helping me understand more!
I've used Gurobi (and other solvers) in the past, but always in situations where we just needed to find a solution that was way better than what we were going to find with, say, a heuristic... I've never needed to find a provably optimal solution...