My results have been inconsistent, often chewy and fibrous, whether the recipe I followed was Greek, Spanish, Italian or Japanese.
The recipes themselves are all over the map with their advice for making octopus tender. Salting is essential to tenderness, or fatal; brief dips in boiling water tenderize, or long slow cooling, or a rubbing with grated daikon, or the addition of a wine cork to the cooking liquid.
Last week I stumbled on a Greek food scientist's report that small amounts of vinegar tenderize octopus.
A solid lead at last! I decided to revisit octopus and really figure it out.