Toxapex may be a poison type, but it's more of a water type! and water types ain't weak to no ground types!
Toxapex is still a poison type and since water doesn't resist ground, its super effective.
It doesn't matter whether it appears to be more of a Water type -- it's still a Poison type, and those are weak to Ground.
Source: Common sense
Hope I helped!
If you sewe the Water type chart, you will see does not resist ground type, and as you say, ground was super effective to poison, so which means it takes x2 damage from it.
Source: You may check toxapex’s pokedex page, the type defense says that ground was 2x to it.
Hope this help!