As a user of both Thunderbird and K-9, I'm glad this is happening. It's probably been triggered by the OAuth2 wows K-9 has had, and how hard that is to maintain (not enough licenses to enable Google's OAuth from GiHub, Google Play Store and F-Droid; each OAuth2 service requiring its own tweaked implementation...).
I'm sure K-9 has recently bled a lot of users because of Google's scary "untrusted app" messaging, which could have been avoided had this agreement, or help, come sooner. But, in the long run, both Thunderbird and K-9 will surely benefit from this.
Free email clients need all the support they can get, because Google and Microsoft are making things really hard for the small players.