I have successfully removed the lap marks, and this answer summarizes my personal experience.
The Manufactuer's Recommendation: Thinner
The manufacture recommended using thinner when I called them about these lap marks. This did not work for me. I tried thinner, first on a cloth making no visible progress, then sponging it to be able to better soak the wood. This was work above my head, very messy, and the fumes were literally noxious. The results also disappointed.
I think the "thinner" approach could thin out lap marks on harder wood (harder than the cedar T&G of my ceiling) and when working on horizontal or maybe vertical surface, for instance on a work bench in a wood shop.
It is also useful to note that the thinner can be applied well after the stain has dried. The stain deposits and penetrates its tint into the wood, where the tint is left behind after the solvent evaporates. The tint does not cure like paint would. So applying thinner can release the tint even weeks later. The difficulty is soaking the wood deep enough to dissolve the tint and then soaking it up with a cloth. With soft and porous wood like cedar, the stain will have carried the tint deeper into the fibre making later removal very difficult.
My Preference: the Sanding Approach
What did work well, as proposed in the other answer, is to sand. You don't have to sand bare. The trick is to sand lightly so that some of the stained surface of the wood is removed, thus lighting the lapped areas.
If necessary, first mark of the darker and/or lapped areas by observing it from afar under different angles. When it comes to ceilings, what you see with your nose on the wood standing on a ladder or scaffolding is different from what your critical family members might see from the floor under different angles. And while working a ceiling area it's good to get continuous feedback from someone down below.
It is also a good visual distraction to mark off full lengths of board. So if there is a seem of butting boards nearby, continue to that seem. The most unappealing effect of lap marks is when they lign up across boards. Breaking it up with lighter runs helps too, even if the tinting along the board doesn't precisely match with the surrounding boards.
Mark the area that needs treatment. View and verify from different angles.
Sand it with light pressure using coarse grit sandpaper wrapped around a sanding sponge: the stained cedar dusting will otherwise quickly clog the sandpaper. Do not use a wood block or just your fingers. I used 60 grit. Work in lengths of the grain, and if possible feather the sanding action out beyond the lapped area, but work the lapped section meticulously.
Apply very lightly some of the original stain on a lint free cloth (e.g. piece of old bedsheets) and rub it in where you sanded. This removes the sanding dust and deepens the tint by removing the whitening caused by abbrasion on the surface of the stain.
Some of the wood will have a strained look (lighter strands of wood fibre stand out) due to the sanding, and this step also evens out such strand discolouring relative to the surrounding area.
Do not be afraid to redo sections. It took me sometimes 2 or 3 repetitions of sanding/staining to get it right. Always check from a distance.
I accepted the earlier answer suggesting the sanding approach as the best answer, and I am providing this answer of my own to share some more details and perspectives.