If most people use 20% of the features of an app, and you create an app that covers these most popular feature really well I think it's fair to call it an alternative. It doesn't say "full clone" or "feature parity".
I could not seriously pretend of a 'feature parity', if I can match 10% of features, that should good enough for most users. I'm not expecting the next Marvel movie to get its SFXs done with PIkimov.