Coppicing doesn't kill the tree. It's a prehistoric method of managing tree growth. It encourages growth, just as lopping the top of a hedge encourages outwards growth.

Cut the shoots back (or, off, in this case), and the stump is encouraged to grow more new shoots.

It's been going on for centuries, and is one of the best forms of woodland self sufficiency around.

The article mixes two things: Growing willows into furniture and turning driftwood into furniture. But yes, the seat is harvested, not transplanted. Still-growing furniture would be a heavy burden in maintenance.

Especially in your living room.

Would be really cool though. I hope they start doing that in addition to the harvested ones.

Totally! It looked like the chairs were grown upside-down though, can kinda see why given the topology of most chairs vs tree topology, but hopefully just a bit more and/or stronger training and coppicing, seems like it should be possible to do a right-way-up one.