I'd ask about xsd:strings and xsd:double too, but I'm not even sure if their definitions in ECMAScript are equivalent to the XSD ones.

xsd:boolean:

3.3.2.1 Value Space
boolean has the ·value space· of two-valued logic: {true, false}.

ECMAScript boolean:

6.1.3The Boolean Type
The Boolean type represents a logical entity having two values, called true and false.

RDF:JSON:

The value space
is the union of the four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays) from [ECMASCRIPT]. Two JSON values A and B are considered equal if and only if the following is true:

1. If A and B are both objects, (...)
2. Otherwise, if A and B are both arrays, (...)
3. Otherwise, if A and B satisfy the Strict Equality Comparison defined in Section 7.2.15 in [ECMASCRIPT].
4. Otherwise, A and B are not equal.

If I were to guess, I'd guess they are disjoint, because that would be similar to the handling of xsd:hexBinary and xsd:base64binary. On the other hand - the definitions look equal, and nothing says explicitely that they should be disjoint.