"Ideas are worthless" means that having some idea about how to make some money is cheap and worthless. It has to be instantiated quite laboriously with a lot of additional effort and lots of other ideas supporting the original core one. You can tell this happens because ten startups may start with the same "idea" but end up with completely different concrete solutions on different platforms with different tradeoffs and each taking ten man-years of work to create. It's in those ten man-years of work that the value lies, not the core idea.

As proof that is where the value lies, many solid businesses exist that have done the work but haven't even hardly got an "idea" the way startup founders think of it, just exploited opportunity. What's the "big idea" behind a graphics design firm, or a plumbing company? On the other hand, try making money with an uninstantiated idea. Where the value comes from is pretty clear.

Now that you've presumably done some of that work, the core original idea remains as useless as ever, but the work done around it to get it some distance down the path to instantiation is not. A new idea would be starting back at square one, and once again be, yes, worthless.

I doubt that he really wants the idea so much as he wants to move forward with the instantiation.

(Ideas with no instantiation effort put into them are worthless because basically supply is effectively infinite.)