One possible explanation is this: Pardo has a learned that there is a high percentage of success in foiling the nuclear program and wants to keep Israel within a low profile. Alternatively, the Mossad may have intelligence that assures to a great extent that the USA will act militarily. In that case, it is better to extricate Israel from the conflict in the international arena, by saying it's not Israel who has the urgency. It may be prophilactic. I would have liked to hear his definition of "existential".
Turkish warplanes attack PKK targets after soldier killed in attack (Reuters)
from the article: Mossad chief: Nuclear Iran not necessarily existential threat to Israel