Seems to be that the Animal Crossing mobile game (and Fire Emblem) will be free-to-play and not include in-app purchases. Pray this will be the case
https://mynintendonews.com/2016/05/11/animal-crossing-and-fire-emblem-mobile-will-be-free-to-play/
In-app purchases is what free-to-play basically means though. Like that term was essentially created as a nicer way of saying "game you can download for free but pay money through in-app purchases". This is actually just a confirmation that it will be an in-app purchase hell as far as I can see.
This is probably why Nintendo in their PR has started using the term "free-to-start" instead of "free-to-play" as it's more genuine of what it really means. (as in that they expect you to spend money later, it's just free to download) See: https://www.nintendo.com/games/free-to-start-games
If this was coming from Nintendo PR messaging and not from a DeNA official through WSJ, I'm sure they'd have used that term instead and I'm sure that's what they'll call the game later this fall.
EDIT: Actually, they did say free-to-start in the actual article:
“‘Fire Emblem’ and ‘Animal Crossing’ are both free-to-start apps,” DeNA Chief Executive Isao Moriyasu told The Wall Street Journal.
WSJ and other sites (including my own) are just saying free-to-play because it's the more widespread term.