I suspect though that a lot of the secret behind Chartreuse isn't just the recipe, but the actual sourcing of the ingredients.

Presumably the recipe relies on very unique and location-specific herbs to the alps. Part of the justification for limiting supply is concern for the environment and sustainability of their production. The order also had to cease production while they were evicted.

I wouldn't be surprised if some of the key ingredients weren't wild foraged or at least very unique species.