Just thousands of regular people manually downloading the WhatsApp client for iOS
Not trying to discourage people from doing this, but they likely have caches or rely on CDN's that wouldn't really show any impact of any kind. Even if they do, a single day of being "down" is not doing much. For it to be effective people would need to continue this for weeks, and I don't see anyone caring enough to do that— they're not even willing to ditch facebook/twitter/whatsapp, we think they're going to maintain a DDoS attack?
Same issue with uploading encrypted backups to Amazon Drive/Google Drive/OneDrive, etc; there's already built-in limits that protect them, and at the end of the day you're just using what you already paid for. If you fill up your storage you have to pay them more to use it. Not a good strategy IMHO. The only real way to hit their wallet is to leave their ecosystem.
The guy in the video claimed the year is 2020. Any info on this? I agree with what he says, but would be great to have context and a year in the title. Without context this makes it seems like this just happened.