Even an aerogel is .0.2g/cc. That sounds implausible to me but so does the solar sail hypothesis. And really we don't have much information about what an interstellar object would be like.
I agree that the proposed density is too low. Anyway, the aerogels are deigned to support their own weight on Earths, wind currents and some moron poking them. The magical aerogel of this object may have lower requirements.
