My 2 cents TLDR: seize opportunity but based on sound analysis and caution, not blind optimism. I think they are saying something of the effect of:
1. The short trade (The Big Short or similar trades during the housing bubble) happened during a period of market euphoria. I.e. when most investors were irrationally confident and greedy.
2. Instead of sitting out or being fearful (as Buffett's original advice would suggest), the people who shorted the market took an aggressive position. They were indeed "greedy" in the sense of seeking profit, but they did so with deep awareness of the systemic risk that others were ignoring.