Keep in mind that the real issue is not whether the WTBTS is an NGO or not. By definition, it is an NGO, a Non-governmental Organization, just like many other churches and groups.
The real issue is that back in 1991 they registered with a branch of the UN, the DPI, and in doing so agreed to support the goals of the United Nations.
And as this thread indicates, they continue to get involved -as an NGO- with various events and organizations, many of which are clearly of a political or interfaith nature. Such "worldly" involvement would probably be very surprising to the average dub, who is repeatedly told that as a JW, they must be no part of the world and be politically neutral.
No Apologies