They can't, if you launch the thing on top of something that looks like a nuclear ICBM and follows a ballistic flight path like a nuclear ICBM does. If that happens, the person on the other end is likely to jump to the conclusion that it is a nuclear ICBM, even if you insist it isn't. Which puts them on the spot to launch their own nukes, if they have any, before yours arrives. This "discrimination" problem has been a big one for the Prompt Global Strike program since its inception.
The vehicle in the story is part of the Air Force's attempt to solve this problem, by delivering the PGS payload via a spaceplane that flies in a flatter suborbital trajectory rather than an ICBM's higher trajectory. This is supposed to make it clear to the people on the other side that whatever has just been launched at them is not a nuke. It's unclear how successful this would be, though, since the spaceplane still has to be launched on a missile (possibly triggering any missile-launch early warning systems the opposition has), and nobody knows if in the heat of a crisis different angles of trajectory would be sufficient to reassure an anxious decisionmaker that they're not witnessing the beginning of a nuclear first strike.
The vehicle in the story is part of the Air Force's attempt to solve this problem, by delivering the PGS payload via a spaceplane that flies in a flatter suborbital trajectory rather than an ICBM's higher trajectory. This is supposed to make it clear to the people on the other side that whatever has just been launched at them is not a nuke. It's unclear how successful this would be, though, since the spaceplane still has to be launched on a missile (possibly triggering any missile-launch early warning systems the opposition has), and nobody knows if in the heat of a crisis different angles of trajectory would be sufficient to reassure an anxious decisionmaker that they're not witnessing the beginning of a nuclear first strike.