Why did Airbnb need to lay people off despite a new round of funding?
In the face of the crisis, it was important that Airbnb as a company was going to survive. Although it was so painful for Brian to let go of the people, there were many more employees and customer that depended on Airbnb's future success.
No one knew how long the crisis was going to last, and the new round of funding was simply a bandaid meant to help in transition. It alone was not going help without serious cost-cutting.