Correct... the article does not explicitly mention the 20-second recommendation, which was presumably arrived at by experimentation. But it does explain why such a time-based recommendation might work:

> The skin is quite rough and wrinkly which is why you do need a fair amount of rubbing and soaking to ensure the soap reaches very crook and nanny on the skin surface that could be hiding active viruses.

It's a complex balance between the effectiveness of the behaviour and the willingness of people to perform it. That's the usual reason, anyway.

So long as you're not stripping away layers of skin, I don't see why 30 seconds wouldn't be better.