Probably just to cross a bullet point somewhere. Still, with their warchest, they could easily dump money on a small company and get it over with. At least something basic, but usable.
Say, timeline plus chat.
The reason it is fast though, is it is not loading that webview from a website, but building it from data locally. That gives it a native app speed.
On the other hand, the original Google+ release for the iPhone was bad because it was pretty much a webview downloaded on demand too.
Facebook is just loading it on demand basically as a webpage.
So it's possible Twitter is using a webview, but downloading the data separately for speed.