A lot of secondary railroads all over Europe are not electrified, and that's where these diesel-powered Lint-trains show up. So that's the target market for these trains, not the main railroads.
And as said by others, there's a big problem with the fluctuations in energy output from wind and solar, especially in Germany. Instead of just throwing the energy away (like they do now), they could just as well use it to create hydrogen, even if it only has 50% efficiency.
That's why they're targeting the German market with this train.