enumerate

enumerate(n) returns a list of integer numbers from 0 through n-1
enumerate(0) returns empty number list {}.
enumerate(1) returns {0}
enumerate(2) returns {0,1}
enumerate(3) returns {0,1,2}
enumerate(4) returns {0,1,2,3}
and so on.

In your example the index is not returning as expected.
Enumerate returns integer list starting from 0 and array/list index starts from 1 in Appian. When index() gets invalid index (<=0) is returns whatever listed as default value.
index({10,20,30,40,50,60,70},enumerate(5),{}) = index({10,20,30,40,50,60,70},{0,1,2,3,4},{}) returning invalid list of numbers {10,20,20,30,40}
index({10,20,30,40,50,60,70},enumerate(5) + 1,{}) = index({10,20,30,40,50,60,70},{1,2,3,4,5},{}) returning valid list of numbers {10,20,30,40,50}
index({10,20,30,40,50,60,70},enumerate(5),1) = index({10,20,30,40,50,60,70},{0,1,2,3,4},1) returning valid list of numbers {1,10,20,30,40}

index({10,20,30,40,50,60,70},enumerate(5),null) returns valid list of numbers {null,10,20,30,40}

{10,20,30,40,50,60,70} is auto casted to List of Number (Integer) where a {} is casted to List of Variant. When returning index always returns the type array input. Merging list of variant and integer ending up with unexpected list.

enumerate(n) returns a list of integer numbers from 0 through n-1
enumerate(0) returns empty number list {}.
enumerate(1) returns {0}
enumerate(2) returns {0,1}
enumerate(3) returns {0,1,2}
enumerate(4) returns {0,1,2,3}
and so on.

In your example the index is not returning as expected.
Enumerate returns integer list starting from 0 and array/list index starts from 1 in Appian. When index() gets invalid index (<=0) is returns whatever listed as default value.
index({10,20,30,40,50,60,70},enumerate(5),{}) = index({10,20,30,40,50,60,70},{0,1,2,3,4},{}) returning invalid list of numbers {10,20,20,30,40}
index({10,20,30,40,50,60,70},enumerate(5) + 1,{}) = index({10,20,30,40,50,60,70},{1,2,3,4,5},{}) returning valid list of numbers {10,20,30,40,50}
index({10,20,30,40,50,60,70},enumerate(5),1) = index({10,20,30,40,50,60,70},{0,1,2,3,4},1) returning valid list of numbers {1,10,20,30,40}

index({10,20,30,40,50,60,70},enumerate(5),null) returns valid list of numbers {null,10,20,30,40}

{10,20,30,40,50,60,70} is auto casted to List of Number (Integer) where a {} is casted to List of Variant. When returning index always returns the type array input. Merging list of variant and integer ending up with unexpected list.