Can we please decide on what the definition of engagement is? For social apps on facebook it is daily users / monthly users. Here it means "more users using the service". Let's not confuse vanity metrics with relative metrics.
Measuring engagement as daily users / monthly users may make sense for Facebook. However, for a company like WePay, engagement should be measured as the number of users who are actually taking action that involves a monetary transaction, because that is what makes the company money (and this is the engagement metric that the article states WePay is using).