Is it acceptable to omit the main clause in 只要～ phrase?
Ah. I think the tone matters here ( from MY own perspective).
When we read 只要……就 sentence, you may perceive that the stress is on 只要…… part. We are emphasizing the condition and 就 part is not as strong as the condition （就 weakened the tone）.
However, here Kim Jong-un is making a  vow that North Korea will NEVER USE NUCLEAR WEAPONS AND ......, unless she is threatened by nuclear annihilation (this is more like a supplementary condition). So 就 is supposed to be omitted to enhance the momentum.
PS. You may let a native read the sentences with and without 就 for you and try to feel the subtle difference.
试比较：

只要你不在此捣乱，我（就）决不撵你出去。( There could be a 就，but I prefer not.)
只要你不在此捣乱，我就不会撵你出去。