This was built with money from an grant from Mozilla. See the bottom of this page: https://www.thunderbolt.io/announcing-thunderbolt
Took me a bit to find, so here's it quoted:
> Thunderbolt is funded through a dedicated investment from Mozilla and is being developed by a separate team focused on enterprise AI products, distinct from Thunderbird’s donation-supported consumer product work.
Mozilla gave Mozilla a grant, so that's all there is to it I suppose.