Actually an AssignmentExpression can be any expression, it's the expression at the top most level, the one that contains all other expressions (see the spec).
However,
export default = { some: 'thing', other: 'thing' };
really isn't an assignment expression it's a syntax error. If you use the assignment operator in an assignment expression, then you need a left hand side and a right hand side. So something like this would be valid:
export default foo = { some: 'thing', other: 'thing' };
Just using an object literal should be correct, because, as I said, an AssignmentExpression can be any expression.
So Esprima seems to be wrong.