Sounds like the Russians decided to design it as a primary mode of delivery.
If they did not get out of the atmosphere macht schnell, I can understand the weird flight anomalies they had on recordings of test flights.
It’s not the only weapons system that uses such a trajectory, but such a launch really requires a missile designed for it. It was reportedly possible for our Tridents to do it, but Tridents weren’t happy about it and weren’t real accurate in that mode.
They’re also harder to hit for ABM defenses because they’re ‘above the horizon’ for a lot less time and are moving a lot faster.