Nobody said it would have a different software library, and in fact that would go against all of Nintendo's statements for the past few years.
He didn't explicitly say that, but if it's a 3DS successor then a separate software library is implicit. Because anything that would replace the 3DS as a true throw-it-in-your-backpack portable will be more performance-constrained than the Switch in order to maintain battery life. So either devs will be expected to significantly downgrade Switch titles somehow, or they'll be creating separate content. The latter seems more reasonable.