I don't think there's a inviolable ratio between active users and engineers, but you do absolutely need more engineers as your user base increases: you need more people on call, more people maintaining the servers you're running (especially if, like Twitter, you run your own DCs), more people doing QA and reliability engineering because of increased load, etc.
It's absolutely an OKR which implies higher workloads, are you kidding me‽ I'm sure you can build some architectural rat's nest where you need 100 engineers to maintain systems that barely serve a dozen requests a second (kind of like the blockchain) but that doesn't mean you couldn't do a much better job.
Everything else being equal, sure, more traffic is harder. But everything else is generally not equal. And despite your sleight of hand in this comment, your original claim about what's "delusional" was based on asserting an equivalence between the WhatsApp and Twitter workloads.
