>> released from an aircraft seconds before its Northrop Grumman scramjet (supersonic combustion ramjet) engine kicked on.

No. The scramjet would only work at supersonic speeds. The missile was boosted to speed/altitude by another conventional rocket before lighting the scramjet.

>> The HAWC vehicle operates best in oxygen-rich atmosphere

As opposed to a oxygen-poor atmosphere? Mars? Venus? And "best" suggests that this missile might operate poorly, but operate nevertheless, in some other non-oxygen place. Maybe this thing also travels through space. We may be going to war on Venus after all.