Why paint fails on wood outdoors, and pigment doesn't
Paint is a skin on a thing that moves. That is the whole problem.
Wood moves
Wood is a bundle of straws. It takes in water through its end grain and swells; it dries and shrinks; it does this every day between dew and noon and every year between spring and August. Paint and stain are a film on top of that bundle. The film does not swell and shrink at the same rate, so it cracks at the microscopic level almost at once, and at the visible level within a season or two. Water gets under the film. Sun gets at the wood through the cracks. The film lifts. You sand it and start again.
Sun breaks fibres
Ultraviolet light breaks down lignin, the glue that holds wood fibres together. That is the silvering: the surface fibres, unglued, go grey and fuzzy and eventually wash away. A UV-blocking stain slows it. Nothing stops it, because the stain is thin and the sun is patient.
HDPE does not move
High-density polyethylene does not absorb water. Not a little; effectively none. So it does not swell, shrink, split or rot, and there is nothing inside it for frost to freeze. It expands slightly in heat, as everything does, and the chair is built with that in mind.
Pigment is not a skin
The colour in our boards is mixed into the melted plastic before it is extruded, so it is the same all the way through: cut a board in half and the middle is the colour of the outside. There is no film to crack and nothing to lift. A scratch is the same colour. The UV stabiliser is mixed in the same way, so the sun never finds an unprotected layer.
What actually happens over twenty years
Very slowly, the surface of any coloured outdoor material shifts a little as the top few microns take twenty years of sun. On pigmented HDPE it is even, it is slight, and it is the same on the arm that faces the sun and the back that faces the wall. That is the difference between a chair that ages and a chair that fails.