If you have no idea where you're going, and no metrics at all, how do you reasonably pick any tasks?

So, clearly, you have some goal. It's likely also measurable. If you pick it so specific that you can't stick to it, yeah, OKRs are nonsensical. But "make money" or "increase conversion rate" or "find market fit" are clearly meaningful goals, no?

The difference from waterfall is that OKRs don't prescribe a "how". They describe a desired outcome. Guardrails within which you can be as agile as you want, just get some results.