Yes - and this is commonly misunderstood. The critical thing in getting a promotion is the ability to demonstrate the capabilities that will be required in the new position. This means actually being able to point at examples of you doing that successfully. This is really different from being brilliant at your current job.

I think you misunderstood the post your replied to. That poster was talking about employees that have been going above and beyond their current role and taking on many/most of the responsibilities that their "promoted to" role would entail. The promotion is just becomes a recognition of the increased capacity that they are currently taking on.

That’s still just one sample outside their expected competence. Ideally, you’ll want to test people over a long time period to take as many samples as possible.