You're right. However, you can ask (same for FX brokers, in my case, too) for verified and stamped official year-end statements so you can attach them as proof of income in your tax declaration. That should be enough. What I get is an official pdf which I just print out (some︀ brokers will send you physical docs by courier at an extra cost), and I keep︁ the contact details of the sender ready should the authorities ask for them to verify︂ the statement(s) ... which they NEVER do.
NVO