This is just slightly misleading, although the rest of the explanation is very thorough and helpful. A statutory heir -- as the term suggests -- is defined by statute and has nothing at all to do with being named in a will or even with choice: you either are or are not a statutory heir according to how the law defines that status (the applicable sections of the civil code are 887, 889, 890, 900, and 907). Indeed, the purpose of the definition is to make sure that inheritance and inheritance tax can be implemented smoothly without the need for a will. Residence is irrelevant (not mentioned in the statutes = irrelevant), and statutory heirs are always included in the basic deduction regardless of whether or not they decide to give up their inheritance rights and whether or not they are actually liable for inheritance tax (which is where residence and nationality come into play).
To this question's OP: When discussing inheritance matters in Japan, you have to be careful to distinguish between statutory heirs and other beneficiaries, even though "heir" is often used loosely for both. Not making that distinction will eventually lead to confusion. Personally, when it comes to transnational inheritance, I wouldn't trust myself to have the knowledge necessary to make any informed decisions unless the situation is really simple. Matters of documentation and the possible applicability of "home country" law are likely to pose a challenge for any nonprofessional.