Doing a big of belated damage control, a new post on Capcom's official Unity blog (thanks: VG247), explains that the additional content discovered by modders on the game disc, including a new extra challenging difficulty mode, and new character costumes and taunts, are all attributable to planned free updates, and not the nefarious on-disc DLC that they have been criticised for in the past.

...some data of this update is on the disc, but said data is incomplete and requires the actual download to access. But again, free. Mercenaries costumes are residentevil.net unlocks. Simply register for the site and start earning RE points for those goodies.

Future paid DLC - like the multiplayer modes we revealed a while ago - is not on the disc. We said as much a month ago, but that point may have been lost in the shuffle.

The tl;dr version is - RE6 on-disc DLC is free, paid DLC is not on the disc.

So there you go: unfinished free content, and regular old achievement-based unlocks. Not so treacherous after all.