Building Twitter/Mastodon *not at scale* isn't that hard and certainly doesn't take 200 person-years. Building it *at scale* is a completely different story. Remember the fail-whale? That was years of Twitter struggling to scale their product.
That said, as we described in the post our implementation of Mastodon is less code than Mastodon's official implementation. So not only is Rama orders of magnitude more efficient for building applications at scale, it's also much faster for building first versions of an application.