And songwriting is not the same as performance. Never has been. Never will be.

Just off the names given you've said yourself that "+ The Machine" and also 'the Night Sweats" make 'Florence' and 'Nathaniel Rateliff' both members of bands instead of solo acts.

The White Stripes was a band (albeit with only two members) while Jack White as a solo artist is a different concept. He also made music with The Dead Weather and The Raconteurs, both bands in which he was a member.

Micheal Jackson was a member of the Jackson Five before becoming a solo artist. Nobody ever thought he wrote and performed all the music, but there really aren't many people (outside of those in this post fighting against gravity) who would say he wasn't a solo act.

If Nick Cave releases a song with The Bad Seeds it's a group effort, if it's released under his name alone it's as a solo artist.

There's thousands of examples.

Edit- and I really don't know much about Bon Iver, but I probably ought to.