So assume it’s an order of magnitude more, or less, dense with zero evidence to make that assumption and draw a bunch of unfounded conclusions based on that. That’s a plan!
Where you don’t have direct data you make the most reasonable assumption available to you. That’s the one they made.
But we do have direct data, not directly of its density, but the composition of its gaseous emissions.
The spectrography of its tail and halo suggest that the composition of this 'comet' is very different than other comets.
We may have to come up with another term for interstellar objects of this kind. Its makeup and behavior are very different. Or maybe our understanding of celestial mechanics are incorrect.
3I-Atlas raises a lot of questions.