Apple does not sell your data to a taxi app. Apple does not transmit your keystrokes.
FluffyChat, according to their privacy page, uses FireBase for Push notifications. If the message content is transmitted through them, and the taxi app uses firebase too, they could—theoretically—associate your data with both accounts and push you ads for another service on behalf of the taxi app.
It’s probably just a coincidence… and I’d be alarmed too, but I’d start blaming other companies before Apple, as they tend to horde your data for their own use, not sell it. Apple hates sharing.