Some ships provide shuttles to town, some don't. Hotels in Reykjavik are mostly in the same area but not all are in the central area, so with the info you have given I don't think we can answer your question.
I'd inquire about Holland's practices on their board.
You can always take a taxi in Reykjavik or ask the tour provider how they suggest you get to the pickup point. I'm surprised a private tour wouldn't meet at the ship, though
BTW, sunset is 10:30 in early August in Reykjavik, moving up to 8:50PM by the end of the month