This link is very vague: they only talk about a “global data panel” without saying how it’s constructed. You can’t build a representative sample if you don’t have an idea of what the global traffic might be like. See also https://blog.alexa.com/alexa-panel-increase/:
> our traffic estimates are based on data from our global panel, which is a sample of millions of Internet users using one of many different browser extensions. However, we don’t just rely on browser extensions. We also gather much of our traffic data from direct sources, including sites that have chosen to install the Alexa script and certify their metrics. It is this unique combination of data from our global panel, plus data from directly measured sources, filtered through our advanced statistical models that allow us to provide you with robust and comprehensive metrics.
> our traffic estimates are based on data from our global panel, which is a sample of millions of Internet users using one of many different browser extensions. However, we don’t just rely on browser extensions. We also gather much of our traffic data from direct sources, including sites that have chosen to install the Alexa script and certify their metrics. It is this unique combination of data from our global panel, plus data from directly measured sources, filtered through our advanced statistical models that allow us to provide you with robust and comprehensive metrics.