Hello Kev,
although I cannot test it, I'm quite sure that this license is just for older hardware. In the past, Juniper required an additional license for IPv6 for some hardware, like the old M-Series or EX series like EX4200.
Today, IPv6 is broadly deployed, and as far as I know, there is no license need for this anymore. But yes, the Juniper website is confusing as it states that this is necessary for MX series Routers.
For MX204, there are 3 license types: Base, IR and R. You can find more information about them here:
https://forums.juniper.net/t5/Routing/What-s-the-difference-between-MX204-MX204-IR-and-MX204-R/td-p/462631