I'm sure 3rd party app usage is low. It's probably only a tiny minority of users who even know they exist. They're betting they can drive however many of us do use 3rd party apps back onto the 1st party one, and increase ad impressions at a time where social media advertisers are pulling away from Twitter.
Ad revenue going up would mean stock valuation increasing, and that bag they're grabbing getting bigger.
Once they go public, it won't matter. They'll have already cashed out. They'll be playing with somebody else's money then.