First off, yes, Nintendo wants 3Rd party support. About 25% of Wii revenue came from 3Rd party royalties, and 3Rd party games sell systems.
Moving on, NX doesn't need to be the most powerful system to get 3Rd party games, it just needs to be as powerful as the least powerful platform that is also getting 3Rd party games. Rumours have NX running the next Tegra with about 90% XBox One power. So that should be enough.
I think Nintendo's idea is to push as many of these things out to handheld buyers as quickly as possible and push the system with their own top titles. Third parties would then see potential new market for their games and port the XBox One version of their titles. It could work, provided the device itself is seen as cool and appealing to buyers.
