The word "existential" (attested since the late 17th century) was used as an adjective relating to existence long before the birth of Existentialism. In the phr

The word "existential" (attested since the late 17th century) was used as an adjective relating to existence long before the birth of Existentialism. In the phrase "existential threat", there is no cause of confusion (unless there is cause to speak of threats relating to Existentialism). So I don't have a problem with this usage.