Earlier quoted context omitted.
You're talking about implementation details on thw server side. Why should the client care about that? Why should the error code for /product/foo/thumbnails/123.jpg be different if served with nginx serving a static file or an application server that dynamically generates it based on the product id?
I make a distinction between the file and the endpoint. While 123.jpg can be found or not the thumbnails/123/ API Endpoint has two parts, the Call and the Argument: 123. To be honest... my argument unraveled in my head while writing - yes you have a point there. So I just can say that 404 is unhelpful because it's unspecific.
But if the route is ID-agnostic, like `/cgi-bin/generate_thumbnails.pl` or `/api/json_rpc.php` then you could justify a 200 OK that the endpoint itself was found, regardless of the parameters. In this design the RPC is definitely above the HTTP level.