For example, you can have a List of type List<string>, which would be a list of any length containing any string.
On the other hand, a good tuple implementation (especially a typed one) could allow tuples like (string, integer, boolean), i.e. only arrays of length 3, where the first positions holds a string, the second position holds an integer and the third position holds a boolean. So yes, usually they are represented as arrays, but on a higher level, they are fundamentally different.
As for unions, the parent comment is not referring to the Set operation, he's referring to Union Types.
With this concept, a variable can be one of many types. For example: var input: string | number And at Runtime you would have to find out whether it is a string or a number, to be able to use it.
(Not here arguing whether Dart does or does not have these language features, just trying to clarify this misunderstanding)