Since Alamut had been levelled by the Mongols some decades before the time Marco Polo arrived (in fact, if he had tried that travel before the Great Khan had conquered most of Central Asia, the Middle East, China, the Caucasian Mountains and Russia, he'd have likely been hanged, waylaid by bandits or imprisoned several times over on such a trip) any information about the Assassins coming from Polo are to be treated with about as much credibilty as the writings of mainstream Muslim scholars on the group, ie not a lot.
Also, if you understand Nizari theology, then it is pretty obvious that the "nothing is forbidden" translation is the accurate one. The Nizari cult believed that when the end times were near, all rules which Muslims were bound to live by would be abolished. This is the Qiyamah doctrine, which was initiated by Sheikh Hasan 'ala Dhikrihi Al-Salam, and the likely origin of various stories about the Assassins and use of drugs, wine, women and beautiful gardens.
The thing is though, Hasan ibn al-Sabbah was himself a highly pious and austere Muslim, albeit of a heretical school, and would not have agreed with Hasan's interpretation of the end times. At least, not without Allah himself telling him it was alright. Therefore the attribution quote itself is almost certainly an invention, likely of 19th century British officers who were present in the Kingdom of Persia and carried out many excavations and explorations as part of the Great Game with Russia. Overwhelming British paranoia about both Russian intentions in the region and the perfidy of central Asian despots may have lead to appropriation of the later day Qiyamah doctrine in order to tar all Muslims with this exotic, yet untrustworthy label.
