The attack did not come from “an enemy of the United States,” or “an opposing armed force,” or a “hostile foreign force,” or an “international terrorist attack” (unless the Secretary of the Army finds otherwise), nor were the injured and killed troops serving outside the U.S. as part of a peacekeeping force.
It’s worth noting that the (admittedly early) reports now say the perp was in the same congregation as (and thus theoretically in potential contact with) some of the 9-11 hijackers, and has spoken positively about violent jihad for years. It is possible that this will be counted as an externally-influenced attack. Whether that’s sufficient for a Purple Heart is beyond my ability to guess.