I'm not familiar with Twitter, but I've been in this situation before elsewhere. An independent developer can bang out a good enough clone for the app that work

I'm not familiar with Twitter, but I've been in this situation before elsewhere. An independent developer can bang out a good enough clone for the app that works in 85% of the use cases. Most users would never know the parts that are broken. An internal developer can't ship that app. The 15% of the cases where it doesn't match the desired business intent torpedo shipping it. Keep in mind the alternative option to the native app is using the web app which likely works in 99% of cases. It's unacceptable to whatever section of the company doesn't have their feature in the native app (but do in the web app) to 'lose' users. Of course that last 15% of the features leads to a doubling of complexity.