As there is no Luxembourg forum, I will ask my question here. I understand the EU is opening to US but each country can make their own restrictions. If I am reading the information correctly, Luxembourg is not allowing US citizens and others, into the country until September 30, 2021. We are scheduled for a bike and barge in early September down the Mosel River, starting in France, Luxembourg and finally Germany. Does anyone have information on this? Am I interpreting this correctly?
The Luxembourg government website indicates that you can’t enter before the end of September, so your interpretation is correct and it appears that unless their policy changes that you won’t be able to enter. I would contact your trip organiser.
The advice to contact your tour organizer is excellent. They could easily do a work around where you don't enter Luxembourg, even though the boat floats through it. You could be shuttled by bus around Luxembourg and eliminate that issue. There are probably other solutions that the organizer is able to arrange as well.
I know it would be disappointing to miss a stop on your tour, but it may be less disappointing than having to miss the entire tour.
Travel to Luxembourg by Citizens of the US is allowed currently. The following government site includes the current list of countries allowed, though this could change before your trip. It also has the requirements for entry.
Americans can enter Luxembourg with either a proof of vaccination, proof of recovery from Covid or a negative test.
However, who knows what the rules will be in September.