This article conflates retention of product use and retention of ongoing paying customers without being clear on which it is talking about at any point.

-You can measure how many users use your product for each period – which is usage retention

-You can also also measure how many users subscribes to your product on a continuing basis over X periods – which is paying subscriber retention

Without being clear about what is being talked about, the comparative % figures could be misleading. It also can change how you would evaluate the usefulness (or not) of a product to users.