They use the textured side of the hardboard which when suitably primmed is like a coarse canvas.
If you want to paint on a smooth rigid surface then MDF will be fine, prime with an acrylic gesso as the traditional rabbit skin glue and whiting is not so easily available and is a hastle to boil & mix.
No artists canvases are acid free, that is a term used for watercolour paper not oil/acrylic surfaces.
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