The auxiliary would is in preterite form with modal function, showing that the scenario is a contemplated or counterfactual one, rather than a certainty. The overall construction is called the modal preterite. So the part in brackets is understood.
That would be great [if it happened].
The second thing that the response does is that it does not answer the question directly. Following the maxim: be relevant, you assume that the responder is, by not answering the question directly, trying to hint something indirectly. Namely:
I am withholding comment about whether I will be free.
As other commenters note, the tone of voice might help one to guess why the there is no comment about availability: (a) the person was so excited about the chance and just responded candidly, without thinking about how to be informative, (b) the person would have to extricate himself/herself from other responsibilities in order to go, but truly is willing, (c) the person may simply "not be free" because of lack of interest or unbreakable alternate commitments.