Not incredible at all. The Iranian Kilo boats are quite quiet, and the Persian Gulf is a perfect environment for them. There is an arrogance in the US towards Iran which ignores just how well they did against Iraq during their long war, and despite the tremendous disadvantages they faced and the equally great advantages Iraq held. Americans tend to judge Iran by their experience with the Iraq army. If the Iranians did this, then it is a very clever way of warning the US to not rush to war with them. p.s. there were numerous collisions between US and Soviet subs during the cold war.
from the article: Iran Revolutionary Guards: Unit engraved emblem on U.S. ship