That's an inane take. Even in the absolute worst case scenario of 3% global mortality, how would that possibly result in a 50% global decrease in economic output?
I think the main assumption is the remaining 97% of people are working with reduced output if they are working at all. Just because you didn't die doesn't mean you went to work and put in 100% of normal effort.
People don't have to die, they just have to stop traveling so frivolously. Some people on this post seem to think that people will realize how much they were traveling, and that on the lockdown they didn't travel and didn't need to.
My feeling is that people will feel caged and as soon as they think it's safe, they'll start traveling again and there'll be a giant surge of travel for a couple months, and then it'll basically return to the way it was.