Yeah, you’re totally right.

All image processing happens locally.

And then it mentions that you can just open the index.html directly, which means it uses clientside javascript or wasm and runs on the browser. You are correct!

On the topic of word choice, you might be right. Save or open might be better.

Just because its run locally, doesnt mean it still isnt a web server. The software could run anywhere and be accessed over the network. The image optimization happens at the server, so download and upload are more accurate.