That just means you'll qualify no matter what. There's an infographic about how the tax credit sunsetting works for Tesla, which applies to Nissan as well: https://insideevs.com/us-federal-7500-ev-credit-expiry-date-by-automaker-estimates/
TL;DR: If Nissan sells it's 200,000th EV in Oct 2018, then all EV sales in that "quarter" (Oct, Nov, Dec), plus the one after it (Jan, Feb, Mar 2019) continue to qualify for the full tax credit amount. The two quarters after that only qualify for half the tax credit amount ($3750), and the two quarters after that only qualifies for 1/4 the amount ($1875). Thus, only Nissan EV's sold in April 2020 or later will get zero tax credits.