According to the DFDL spec, fn:exactly-one returns "True if the provided sequence contains exactly one node/value," which suggests that the function would return false if the sequence contains > or < 1 node/value.

The XPath description states that fn:exactly-one "returns $arg if it contains exactly one item. Otherwise, raises an error."

It seems of value to have a true/false result, because something that raises an error like this can only be used as an assertion effectively. On the other hand, that's not what XPath does.