How is that valid for creating and maintaining Twitter? 15 backend engineers are not even sufficient for any single facet of Twitter. Plus, there are other initiaves that Twitter takes on that are probably outside the scope of what you consider Twitter. Take a look at just their open source initiatives: https://opensource.twitter.dev/projects/. Go to, e.g., Finagle's Github page and look at the number of contributors and commits. While you may disagree that all of this is essential for creating and maintaining Twitter, there are problems that creep in when your platform supports billions of users that are not solved either correctly or at all by existing libraries and services; plus, open source projects serve as important recruiting collateral for engineers, especially for companies that do not have a set of widely used tools that entrench engineers (e.g. AWS, GC, Azure).

Creating a husk of an app that looks vaguely like Twitter and supports hundreds of users is a weekend project for anyone with a modicum of talent; building a platform that supports billions of users and is monetized well enough to support itself is an entirely different beast.