I always assumed a nearly 300 year old body decomposing in non-sterile environment would look a heck of a lot worse that what we see in the episode. Never dawned on me that was a mistake of any kind.
It's almost like the script was meant to portray the dead body as though it was still in good condition, as if they could have had an actor actually in the bed. This would account for Riker's comment:
"Looks like the poor fellow died in his sleep."
And it would have been very simple. Have the actor appear to be resting peacefully but motionless and all the dialogue would fit. But when his corpse is a skeleton with a huge gaping mouth, I'm not going to buy the "died in his sleep" idea. After all, we're in a simulation of a book where mob guys who openly murder people in crowded casinos.
The script specifies, "a skeletal figure -- long dead and decaying." Based on the final look of the corpse in the episode, I think what was intended was a kind of natural, partial mummification like this
Compare that to the body
in "The Royale."
Remember that Data doesn't say, "the lack of any
decomposition" he says, "the lack of any advanced
would be advanced decomposition. So, I don't think it's that far off personally. If Data had also said, in addition to the environment being sterile, that the climate in the building was unusually dry and arid, I would think that what we see in the episode would be dead-on.