It's still a Ponzi scheme though, whether the website is online or offline. Once enough money has been collected or when withdrawal requests start to outpace new deposits, the plug will be pulled. This can be next week, or it can be in many months. Some successful online Ponzi schemes last for over a year before exploding.
Is it too radical to believe they are legitimately and successfully executing arbitrage?
Arbitrage is a legitimate trading strategy and can yield some profits. I know this, because I operate a simple arbitrage bot myself. However, the profits that they claim to achieve are way beyond what you could reasonably expect considering all the funds that are in transit through the slowass bank-system and the immense costs of having 10 employees and an office in one of the most expensive cities in the world (Geneva).
I've written out the above in much more details in previous posts in this thread, so I won't bother to redo that. But if no alarmbells go off when you see this site and its claims, then you need to get your Ponzi-sensors checked.