If joining the UN helped the preaching work then it was justified. No Witness would help the preaching work by joining the YMCA.
Jehovah's Witnesses use the world at the same time that they attack it - this is nothing new. They believe that America and Britain are the seventh beast of Revelation - but does that stop them doing business with them? Of course not!
The whole UN nonsense was a storm in an apostate teacup.