I think what Dallas is actually trying to say is that it is his initial impression that the space jockey is "fossilized" because he, like we, actually think the outter surface of the body as being a mineralized structure, so the whole creature is like a pertrified "mummy" with even the larger muscles and bone turned to stone, rather than as we NOW know it actual is, an environmental suit.
The sad fact is it works brilliantly a mummified alien corpse, much less better as an odd design choice for clothing. Whenever I watch "Alien" all I still see is a strange creature, its eye sockets sunken in death deep into a skull still locked in a frozen scream of agony as something exploded from within its chest cavity.
NOT the other.
And thats the frustrating thing about Ridley , he says he doesn't listen to the fans, he likes to ignore and twist their expectations. Only the problem is he doesn't replace those theories and extrapolations with any better ideas , just poorly thought out ones which, OK THEY were unpredictable, but only because they were on the whole ,stupidly unpredictable. Hands up those who guessed before Covenant that David would be the father of the final Xenomorph?
In retrospect the whole of the space jockey chamber in "Alien" is actually pretty clean and undamaged (apart from the hole in the floor and the jockey) if a little dusty. And of course now we have to guess who, if Dallas and the team HAD actually removed the helmet, is under there. The transmission is garbled and of none human origin but percieved as a warning, presumably for other "Engineers". I kind of doubt its a human, or David but neither does it strike me as likely being an Engineer, unless Ridley twists this again, so that the "Engineers" actually end up saving the human race by wiping out Davids army of Xenomorphs (or what ever else he manages to breed from all those embryo's and colonists).
Like I said I didn't buy this world as the planet of origin, there must be more colonies given the number of destinations in the Star chamber charts. Perhaps , it is the ultimate "son " of David, the perfected species he finally breeds which , horrified by its origins, destroys David and sacrifices itself by going to die in isolation , like the Frankenstienian monster the story so often hints at. And so the circle is completed.