Do we actually know for sure they are already working on expansions?
Yes.
There is a quote somewhere (Tinman can usually find these things) about how they have a separate development team working on the expansions.
Indeed I can.
Our first commitment is supporting the game with regular updates. This won’t just be the fixes and tweaking we’ve provided since release but will also add new features and content. Examples of this are Wings and associated multiplayer features, as well as the additional ships mentioned in recent newsletters. We’re proud of the fantastic game that we released before Christmas but intend to keep growing and developing it. To make this happen we’re expanding the team working on the game so that we can maintain a regular schedule of updates as well as working on paid expansions.
That's from the first Dev Update on the 8th of January. So they have been doing it since at least then...
https://forums.frontier.co.uk/showthread.php?t=94899
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