I used to sell production gear for KM direct. There may be some legitimacy to the dealer's statement. I was working at KM when we switched from the C6000/C7000 series to the C1060/C1070 series of machines. They were a completely different base engine, paper decks, and finishers. For the next 3 series (C1070, C2070, and C3070) the paper decks and finishers remained nearly identical and were mostly compatible between them.
I left before they started selling the C4080 series, but a quick look at the spec sheet on the website shows that most of the accessories now have a different model number. Whenever KM did a major changeover like this, they did a massive push to sell off all existing inventory to clear out space for the new stuff. There was no way to get people accessories for the C6000/C7000's after a short while, unless we happened to pickup one when someone upgraded their old machine. But that stuff didn't last long at all since everyone in the region was looking for the same items. Even when we switched to the C3070 series, the paper decks went from PF-707 to PF-707m. We had a hard time finding PF-707's for used C1070's after a short while.
Looking at the spec sheets between the two, I do see there are some finishers that have kept the same model, however, the biggest problem is the Relay Unit. It is required to attached nearly every finisher/accessory. It was an RU-518 on the
C3080, but it is now a RU-518m on the
C4080 series. Beyond that, two of the common stacking finishers such as the FS-532 and LS-506 have changed to the FS-532m and LS-507 respectively.
Your best bet is to find the accessories online (eBay) or used equipment brokers like JJ Bender as
@gregbatch recommended.