To be fair, Idris does not in general support parametric polymorphism, though you could extend it to do so.
Idris has a quantifier which is called Pi in type theory, and this is not the same as Forall (the quantifier present in Haskell and ML). Forall gives you guarantees about uniform behavior over the domain of quantification (this is parametricity), whereas Pi does not.
Idris does however recover parametricity anyway for types, since it lacks an eliminator for the universe (i.e. it doesn't have type-case). But it doesn't have parametric polymorphism for values.
Idris has a quantifier which is called Pi in type theory, and this is not the same as Forall (the quantifier present in Haskell and ML). Forall gives you guarantees about uniform behavior over the domain of quantification (this is parametricity), whereas Pi does not.
Idris does however recover parametricity anyway for types, since it lacks an eliminator for the universe (i.e. it doesn't have type-case). But it doesn't have parametric polymorphism for values.