The pierced base on this sofa carries the whole weight of the piece and everyone sitting on it, which is a great deal to ask of something that is mostly holes.
The answer is that it does not, quite. Behind the carved openwork band runs a plain, deep seat rail in solid teak - the actual structural member, jointed into the legs and carrying the springs and the webbing. The pierced carving is a separate face, worked in its own thickness and fixed to the front of that rail.
Which means the scrollwork can be cut as fine and as open as it looks, because nothing depends on it. Cut a structural rail into that much openwork and the sofa would flex every time someone sat down; keep the rail solid and hidden behind, and the carver is free.
The two are made to the same length and curve, and the carving is set out so that its densest passages fall over the leg positions - which is where the eye expects to see support even when it is looking at ornament.