e.g. if you support polymorphism/generics in your static typing, there is a domino effect of things like:
"then how do i do X in the type system?"
"why can't i do Y in the type system?"
"how do i debug things in the type system?"
"how do i grok this type system error message?"
"how do i successfully write the type spec i want?"
and if there are not good answers to such things, then it is a DX fail.
(i think this rule is true for all UX, thus also for DX as a subset of UX.)