When shorts are continuing to short, they are by definition not covering. In the situation of approval being days to weeks away, they are digging a deeper hole. When intrinisic value continually grows, to manipulate the price lower, it requires a continually significantly larger quantity of shorting. That is why I believe over the years, their naked/legal short position has grown to become a very large position, maybe a "too big a naked-short position to cover". That is my speculation.
Yes, they could have covered years ago. They could have also decided not to short a unicorn company that provides the foundation of curing cancer, but they did not.