Then they are probably lying!
What triggers the ban IS known, it's going online in-game or with the game loaded (eg: connected to the internet and switching to the browser while the game is running in the background). It's NOT simply having playtime on the game, otherwise all reviewers from sites like IGN would have gotten banned too! Some of the people doing pre-release reviews have said that too, Nintendo told them to simply not go online for as long as the game is running (that includes playing single player but with wifi on). If you play offline, then close the game and go on a different game and play online there, Nintendo will almost certainly not consider you a pirate (although there have been VERY rare occasions). So there you have it, play offline and you will be fine! However, there's also another flag, save files that started before release date are relatively often flagged on the first 3 days of a release, so either start a new game the moment the games are officially released or don't play online during the 3 days after release. The 3 days have been set by Nintendo for a couple games now and are a way to tell who started the game earlier through a possible leak, but after 3 days they stop the flagging since there have been unjust bans in the past just because some people had wrong date on their systems (time zone mix-ups, etc.).
To sum up, don't have the game running, either with you playing or suspended in the background while having the wifi on prior to official release and if you started playing with a leaked copy, either start a new game on release date or wait 3 more days before playing online and you can avoid the bans (NOT 100% certain method though as there can be other unknown factors).
Finally, if the leak comes from a review copy, for the love of god, redownload the game from an actual dump or FreeShop since review copies have different IDs and could very possibly trigger a ban! Review copies are always shipped back to Nintendo, so if they see you playing on one, things can get bad.