Goddammit, I just realized why the study was conducted in 2021 but the results didn't come out until 2024.
https://www.forbes.com/sites/cindygordon/2024/01/30/replika-launches-new-immersive-ai-wellness-avatar-experience/
Replika launched a mental health product at the same time, and I strongly suspect that this is part of their press blitz.