Totally agree. Most companies that have platforms start out building an end user focused app, gain success, and then open up some level of their internals to become a platform. For example, Facebook. It was just an app, and then, after it had success, launched a platform. Even the iPhone initially did not have apps. Amazon is another example. It started as an app and expanded to be a platform that other partners could plug into and take advantage of Amazon's services, particularly logistical ones.
Further, most startups that try to build a platform from the beginning fail, because to have a platform you need a vibrant set of partners who make money off your platform, and it's very rare for a startup to be something that others will build a business on.
Perhaps those that build a marketplace could be viewed as both an app and a platform, like an Ebay. Of course it's necessary to get scale as the marketplace for it to really be successful and provide value to both sides of the transaction (buyers and sellers).
Note that the Amazon example shows how Amazon transformed from an app to a platform by adding the marketplace.