Propulsion and Harpoon anti-shipping missiles are probably what hit the tankers. Quiter than diesel-electric and can stay down for weeks without surfacing.

What makes the Rehav and its predecessor the Tanin so capable and dangerous to its foes is that it uses Air-Independent Propulsion (AIP) to stay submerged for weeks at a time. This opens up an operations envelope that was long only accessible to much more complex and expensive nuclear-powered submarines. Not only that, but the Dolphin 2 class’s version of AIP propulsion uses fuel cell technology, which is extremely quiet even by AIP submarine standards.
https://foxtrotalpha.jalopnik.com/israels-newest-and-most-advanced-sub...