First, if you look at Europe as a whole, you will see that sleeper trains were never really dead. There are plenty of them in Germany and Eastern Europe. Often they go from large cities to holiday destinations and therefore are mainly used by people travelling for leisure.
The novelty of these new lines (be it this company or the transeuropean effort to connect european capitals with sleeper trains) is that they mostly travel business travellers, people who would usually have taken a place. As plane journeys are often the largest item in a company carbon account, they are betting that some will push their employees towards the train.
It might not be easy however. Track capacity at night is often severly limited by maintenance work in France for example and these trains will have to compete with rail freight transport at a time when Europe wants to massively develop rail freight.