The point the article you're quoting is making is that if you insist on a single number then 5 is enough if your question falls in a specific category. Namely, for the purpose of finding the number of people needed to get the biggest payoff in terms of number of usability problems identified.
This being the key point they're making:
> The main argument for small tests is simply return on investment: testing costs increase with each additional study participant, yet the number of findings quickly reaches the point of diminishing returns. There's little additional benefit to running more than 5 people through the same study; ROI drops like a stone with a bigger N.
This doesn't quantify the scale of the reported problems. It goes into it with the assumption that you simply want to find as many of the problems with your site as possible.
> Even if you were doing statistics?
The sentence you quoted regarding this makes the point that the number of users the site has is not the factor that should drive the size of your panel (you'll note that elsewhere the article argues that where you need statistically significant results you need at least 20 users and more if you need tight confidence intervals). Rather the size of your panel should be determined by your required confidence intervals, and when identifying usability issues is your main concern they don't need to be tight.
This being the key point they're making:
> The main argument for small tests is simply return on investment: testing costs increase with each additional study participant, yet the number of findings quickly reaches the point of diminishing returns. There's little additional benefit to running more than 5 people through the same study; ROI drops like a stone with a bigger N.
This doesn't quantify the scale of the reported problems. It goes into it with the assumption that you simply want to find as many of the problems with your site as possible.
> Even if you were doing statistics?
The sentence you quoted regarding this makes the point that the number of users the site has is not the factor that should drive the size of your panel (you'll note that elsewhere the article argues that where you need statistically significant results you need at least 20 users and more if you need tight confidence intervals). Rather the size of your panel should be determined by your required confidence intervals, and when identifying usability issues is your main concern they don't need to be tight.