As I understand it,
given an IDL array type T[],
and a platform array object A that represents a value of that type...

When setting an element of A, [[DefineOwnProperty]] converts
the incoming ECMAScript value to IDL type T.

When fetching an element of A, [[GetOwnProperty]] *should* convert
the outgoing IDL value from IDL type T to an ECMAScript value.

However, in "4.2.24.1. Platform array object [[GetOwnProperty]] method",
step 3.3 says:
    Let value be the result of converting the indexth element of
    the IDL array that A represents to an IDL value of type T.

So I think the phrase:
    to an IDL value of type T
should be changed to:
    to an ECMAScript value
or maybe:
    from an IDL value of type T to an ECMAScript value

(Note that the word "converting" is a link to IDL-to-ECMAScript conversion,
which is correct.)