You miss the fact that their commission is paid one time for the period of 120 days, they lock your deposits for this period. So they pay 5% and use you money to trade 80 days (let us assume there are 80 working days in this 120 days period). Let us also assume they keep 0.2% in profits from the sum you invested, the rest of profits is being paid to other investors. Over these 80 days they make 16% and pay 5% in commission. Over larger amounts, this is a lot of money.

Also, do not consider arbitrage as only BTC-BTC trading process. There is BTC to fiat, alt coins to BTC, alt coins to alt coins. Lots of different exchanges and lots of opportunities. All I am saying that this is possible IMO.