Thank you guys for valuable comments. I have no doubt they keep all the funds in Bitcoin (real funds). They pay back in dollars. One of the reason why they could stay afloat for longer time is because bitcoin price went up. Let's do math.
Bitconnect started around march, 1 bitcoin was around 1000 dollars, now around 4,000 dollars. Let's say I invested 1 bitcoin=1000 dollars. Which is very unlikely, most people started investing in august. But for example sake, lets say I was early investor.
From March till now I should be up around 200%. So my total investment plus profit in dollars is around $3,000. We can add another 500 dollars in compounding. So even even though I made profits in USD, Bitcoin went up at the same time. Bitconnect did not pay real profits to me.