They may have used a combination of physical shares and CFDs as...

They may have used a combination of physical shares and CFDs as their strategy and this moves will not show in Shortman.com and brokers which took this trades will find the shares to borrow from their own holding or other institutions.
We do not know how much capital JCap is willing to put in their trades or others traders and JCap may be trading on some other big funds money.

WTC is not paying dividends so the short trades are not risking paying out dividends like shorting BHP or ANZ.

The selling looks like not finished yet and none of us know when this will stop and feels like the damage is already done.

I know nothing much about WTC but to me it is a trade and opportunity to profit and am willing to take the risk and will hold my short as long as it is still validated by the market.