It was only in 2022 that they decided to merge the projects, and there hasn’t been much on its face to show for it, but I gather that the back-end and behind-the-scenes work will eventually get it to the point that MZLA Corp will feel comfortable giving it the Thunderbird name.
https://arstechnica.com/gadgets/2022/06/email-client-k-9-mail-will-become-thunderbird-for-android/