Correct me if I am wrong. If I install it then I can debug all the issues my customers are facing on my shopify app?

It seems should be fine.

After two reads, this looks like a session recorder for we applications, but descsibed in a convoluted "remote debugger" way, to avoid legal issues.

Maybe someone more knowledgeable can correct me, where is the line between 'collecting harmless telemetry' and 'illegal spying on customers'?

I think it depends on how you use it. It can only start recording after initiating. If you allow your customers to initiate the recording and provide them the option to remove it manually, I think the collection will not be considered illegal then. After all, it is just an open-source tool.

interesting