This appears to reinforce what I wrote rather than refuting it. Mozilla graciously performed the legal legwork of forming an entity capable of collecting donations earmarked for the Thunderbird developers. However, as far as I can tell, this does not imply that any Mozilla developers actually work on Thunderbird (as part of their job at Mozilla, anyway), nor does it imply that Mozilla is setting the direction of the project.

It's a subsidiary of Mozilla. They all work at Mozilla.

It'd be like saying no Microsoft developers work on GitHub or LinkedIn. They all work for Microsoft. They're a distinct group within Mozilla/Microsoft, but they're not outside of it.