You're right, identifying the missiles as Iranian is an assumption.
Here is some circumstantial evidence: "The attack occurred after [...] Iran’s top national security official, wrote in a post on social media that Iran would hit Israel and the United States 'with a force that they have never experienced.'"
That said, it's not exactly a stretch to assume that the missile comes from the country they just bombed. In the other headline, the strike could be Israeli or American.