Tax treaties do not confer benefits based on or reference domicile, which is a different legal concept than tax resident that is usually defined in Article 4 of income tax treaties.

Assuming your client is a tax resident of Germany, you are correct US social security benefits would only be taxable in Germany.  You would report the distribution on Line 5a and $0 on 5b.

Modifying the IRC based on treaty article for social security benefits is specifically excepted by the Regs for disclosure.  Consequently, F.8833 will not be required but I would still include a simple statement to support the exemption claimed.