I don't know the "hows" of the situation, but am told that this is the direction the Union would like to go. They seem to believe that going in the direction of integration is more likely to protect their own situation, than a stance of absolute rejection.
Basically, Norwegian needs pilots, and merely rejecting the newcomers (for whatever reason) seems to have been ruled out as likely provoking an escalation of the conflict (to the detriment of the company) rather than providing a solution.
I think this is a very sensible & prosaic attitude, and probably why those much better qualified than me to judge, have expressed their optimism that things are moving in the right direction.