The marketcap is just a representation and interpolation of the current trade volume.
So it was only theoretically at 820 billion. This number might be way lower.
Tether is also contributing to the fact and you don't really know how much it is actually backed by real money. We might be looking at 1:10 , some people believe its 1:50 or even worse. If you consider this fact it doesn't look too suprising anymore that the market is in depression after 400 million USD worth of crypto were sold.