What stupid mistake? It was an NPM bug.
Yes, but the package on npm is, indeed, corrupt and the latest semantic version.
Anyone creating a fresh project right now with uglifyjs somewhere down the build chain will face this issue.
The only way to resolve this is by publishing a fresh, correct version to npm ragardless of what caused the issue.
The authors have failed to do this till now and the issue remains closed.
It also doesn't help if your attitude is that if you think everyone else is being an idiot you don't stop and check if maybe the problem is with yourself.