I think the headline is fine tbh.

Ikon is doing some pretty cool things and perhaps there's a different nomenclature for 3D printing something as one piece (without any seams or assembly) but is something any less 3D-printed because it was printed in multiple pieces versus a single piece?

I think yes it is less 3d-printed if it is assembled out of multiple pieces.

How small would the pieces have to be before you say it's not a 3d-printed building? Is a building made out of 3d-printed bricks a 3d-printed building or are the bricks 3d-printed but the building not?

The limit as piece size tends to zero is that 3d-printed-ness tends to 0.