The PCInt was formed in 1943, and the tradition that the ICC draws from, the GCF, considers it to have been premature. Certainly though, the comrades in the PCInt had reason to think it was time to declare the party. There were large scale struggles in Italy, the war was coming to a close, and there was reason to expect revolutionary upheaval to follow the was at least in that it had followed the previous one.
Those who expected the war to turn into a civil war were, unfortunately, mistaken. Personally I agree with the ICC analysis that the PCInt was formed prematurely. I don't think that it was a 'voluntarist' move though. I think the analysis of the conditions was wrong. We can all make mistakes.