They said as much in their intro.

Due to a rare defect in our software, sometimes the zip is corrupted.

We blame the user for not backing up, or backing up corrupted files.

That is when I stopped reading.

I think you misunderstood: the point was sometimes project corruption is due to a defect in our software, in which case it is definitely our fault and we race to fix it. But most reports of corruption are bad zip files, which aren't our software (unless you think the widely used and battle-tested library zip.js is writing bad zip files, which I don't think is the case). My best guess is it's faulty hardware or system-level issues.