You can see more context in this thread The end of vactrols
Basically the vactrols that used to be used in LPGs had a lot of cadmium in it. European law banned the sale of electronic equipment that exceeded the threshold of a certain amount of cadmium.
A few makers get around this with their LPGs by creating circuits that have a similar response to using a vactrol but with custom circuitry. Pittsburgh Modular and Rabid Elephant do this.
I’m not sure how Intellijel handles it, but the cost of this module probably means they found a compliant vactrol rather than using custom circuitry.