I don't feel qualified to speculate on the implications for astronomy because that's not my field. However, Celeste is indeed a generative model and you can draw an image sample from the Celeste model and compare it to the original (there's a few examples in the Celeste issue tracker - https://github.com/jeff-regier/Celeste.jl/issues/625 is one I found on a quick look but there's more), so I think it qualifies in that sense. Of course you may want a more sophisticated model than Celeste for a complete model of the sky.