The book was written more for the nobles and shows how out of touch they were. "When the soldiers stand leaning on their spears, they are faint from want of food." or "Make forays in fertile country in order to supply your army with food." are no-brainers.
You're generally on point about the implication that the highest status groups in those times, in that place, were so far apart from the lowest as to be practically isolated. However, I disagree with the presumption that Sun Tzu was simply telling spoiled blue-blood nepo-babies that people die if they are killed.
Every good comprehensive philosophical manual (which is a universal military manual based in Taoist doctrine in this case, the first of its kind) begins with or incorporates certain fundaments to set the stage and guide the reader on how to fill in the gaps of meaning. It was designed to lay out the many common and some uncommon causes of military friction and prescribe remedial action at a strategic level. It is often said that the best strategists act and react to the field of war as if it were their own body, and this is how they achieve it, through doctrine and experience, with cleverness being far from the highest factor.
In the same way you might say a noble officer may not know his men are hungry, a common footsoldier may not know how to thrust his spear effectively. Both of those things seem incredibly basic, but in the heat of battle, a soldier is afraid, confused, dazed, seeking orders, looking for comrades, exhausted, sick, injured, or otherwise frozen. Suddenly, thrusting the spear in an effective manner isn't so simple anymore. The same goes for the officer: When strategizing and juggling multiple units and thousands of men, keeping an eye on the everchanging broad scope of the battlefield with hundreds of variables below and above, seeing a few men leaning on their spears behind a dust cloud on the far edge of the reserve formation might not immediately trigger the implication that they simply haven't eaten, if it even registers as significant at all.
Just like the soldier's life is the proper grip, angle and force of his spear thrust, drilled until he does it correctly and precisely in his sleep, so too is
The Art of War the breath and sustenance of the military officer, such that all factors simple and complex are handled with automatic ease through prescribed action. By the way, for both the grunt and the general officer, things are much the same today...