Watching the video with no other information if looks like the Elbe is just out sailing and decided to cross in front of the Astrosprinter. But the interim report adds a little context.
From the report:
Looks like what is being said here is the Nr. 5 Elbe turned around to go back to Hamburg which evidently required being on the other side of the river. Because of messing with the sails they were late getting across to the other side making the crossing much closer than expected.
In hindsight they should have passed stbd to stbd but they likely underestimated the speed of the downbound ship and also may have expected the Astrosprinter to alter to stbd for a port to port.
I just noticed that a third vessel was involved, the Hanna, sounds like the Elbe just passed one ship close just before encountering the Astrosprinter.