The only thing is that it does say the following on there in bold, not sure what it would entail exactly:
The following stocks are listed on the Reg-SHO watch list. Reg-SHO is a US regulation governing stock shortability. In general, Reg-SHO restricted stocks are not permitted for shorting. However, IB may allow Reg-SHO stocks to be shorted when inventory conditions permit. The main shortable list defines the actual availability