Earlier quoted context omitted.
Or just use http://a.co/gVf3K0x ...
They're talking about generating the shorter link just by looking at the first one (e.g., in your head). How do you get your link out of https://www.amazon.com/Red-4TB-Hard-Disk-Drive/dp/B00EHBERSE... without using a tool?
In practice, it is unlikely for anyone to share an Amazon link without having navigated to the page at least once. Given most sites have this option, it might seem silly to some that one suggests manually shortening the URL in the address bar instead of just clicking share.