But nobody is claiming that comets are bone-dry -- except for one crackpot page I found with lots of exclamation points and misspelled rantings about scientist conspiracies and extended tracts on politics and philosophy.
Then you're not looking very hard. There are plenty of sites with no misspellings, exclamation points, rants, or conspiracy theories. But if a site does not agree with mainstream astrophysics, you
might still call them crackpot.
The point is, most people have no idea how tenuous astronomical models are, yet they are always presented as hard fact.