Why is this significant?  For me there are two reasons.  

1. First it suggests to me that even with around 177 million outstanding shares, that sharesholders have such a tight grip that short sellers have been forced to fill orders with shares they could not even locate.  
2. And perhaps more importantly I see it as evidence of a high degree of motivation by those short selling to keep the PPS in check.  Even when they can't locate shares to borrow and sell, they're willing to "go naked" by selling shares that are not available.  

For those who either missed this discussion or who aren't aware, today's buy in takes the total up over 150,000 in May.  Shares that were sold but not delivered.  The TSX has a facility called TSX-Buy Ins to allow the Canadian Depository for Securities to settle trades where buyers have not had the shares they purchased delivered.  

https://www.tmxmoney.com/en/research/buy_ins.html