Yes, I believe that the moonphase in some Pateks and Langes are said to be accurate for upto roughly 120 years. But in any event, I believe as you suggest they are also more precise and divide the cycle into a very exact number rather than just rounding it to 29 days.
However, I do wonder what they mean by it not requiring any adjustment for 100+ years, and if that is simply based on a hypothetical using their mathematical forumals and going with an assumption that the watch will be continuously worn and powered for all of those years. Realistically, most people will take the watch off at some point and it will lose power and have to be reset. In addition, even if someone wore it non-stop after several years it would need to go in for a service and it might get a little fine tuning.
The owner would not have to make any corrections for short or long months or even leap years for about 120 years. Tried to find the exact time, but was unable. Besides this may vary depending on watch but yes we could be talking about as long as 120 years.
But of course this means it would have to run continuously. This is why they have watch winders, however as you pointed out there would be times the watch would need to go in for a service, and of course this would be one time when it would have to be reset and re-wound. The power reserve may not last long enough until it is actually delivered. But the watch would have been stopped when it was disassembled during the service regardless.
But still a marvel none the less.