Assuming that this is the same ferry that goes from Copenhagen to Oslo, there were no reservations for breakfast so far as I know. I was on the RS Scandinavia tour. We had tickets that we gave to an attendant. The selection was huge. You could stay as long and stuff yourself as much as you wanted.
There were times when the attendant was not there. Toward the end of the service there was no attendant. I took advantage of that and just sat in the bow, drank coffee and gawked while we pulled into port.
I ate dinner at a kind of fancy restaurant, not the buffet. I joined others in the group. I don't recall a line and I don't think they got reservations, but it was pretty early for dinner. I was the last person at the table and it started to fill up about the time I left. Unlike the the website indicates, there were many tables with less than 8 people, but perhaps being part of a larger group counts.