Again, he said this in 2015. NX may release one year later, in 2016. Given two more years, they can release in 2018, as you said. That's 4 years from the release of Tropical Freeze. The same span of time that separated Returns and Tropical.
it wasn't 4 years that separated dkcr and dkctf. it was three years and three months. even then, the game had apparently been finished for the 2013 holiday season and was pushed back when mario kart 8 couldn't make it for its original release date (i assume that would have been march 2014 to beat the fiscal year).
if you add three years to the time dkctf was finished, that would put retro's new game at fall 2016. more than that, they've been on a 'schedule' of a game every three years ever since metroid prime 2 wrapped, even when they're helping other nintendo teams with games.
i fully expect retro's game to be in time for this upcoming fiscal year - march at the latest. they're also the likeliest candidates to be working on a new metroid prime, since next-level's spinoff is a spinoff of metroid prime in particular. tanabe's quote means nothing. it's a hypothetical about what if they started development on metroid in 2015. i don't think he ever said it wasn't actually in development.