What I’m referring to is a military loss typically doesn’t mean you lose all the soldiers, usually 30-50% (even ~20%) is already considered a rout. In this chapter they implied they lost 300,000 men and in the previous chapter the losing side always seem to lost almost the full number of their men. This is exacerbated by the fact that Qin has been waging consecutive wars and would have been constantly losing able-bodied man.
So, IMO, the reality is that the loss is exaggerated firstly by the winner to pad their accomplishments and second, more generally because they don’t have proper record of their people and they may just count those MIA or fleeing during a rout as simply killed.
I recommend reading
The Early Chinese Empires: Qin and Han by Mark Edward Lewis if you want an understanding of how exactly Chinese armies ballooned to the sizes they got to, and why the percent death toll in China seemed to be so high compared to Europe. The tl;dr is that Qin and the other states of the Warring States period were organized along the lines of total war where literally the entire male population was considered eligible for service and where it represented essentially the only form of social mobility. You'll notice that Rome in the same period had similarly gigantic armies; by the time they were involved in Greece, they kept upwards of 200 thousand soldiers mobilized year-round, and this was made possible by a similar system of military recruitment.
The difference is that Rome didn't seem to apply the system consistently abroad (outside of Latium and their Italian socii), while Qin aggressively pushed this system everywhere they went. It got to the point where the land in Qin was organized into rectangles, to better allow the Qin state to hand it out as a reward.
China's logistics were also different from European logistics, partly because of the crops grown and the resulting population density patterns. It wasn't realistic to sustain a hundred thousand people in the field for any length of time if you're relying solely on smallholdings farming wheat, which is why you basically don't see armies of that size in Europe until the early modern era. Without proper logistic chains it wasn't even really possible to sustain ten thousand or twenty thousand people for very long, you couldn't forage (read: rob peasants of) enough food to last, which is why European kingdoms, with their pathetic organizational ability, never had particularly large armies in battle even when their populations were big enough to support them. Agincourt, a clash of two royal armies which represented the full effort of their kings to bring forth their military might, probably had no more than 30 000 people on the field total, and possibly as few as 20 000. But that wasn't because (or solely because) Europe was a depopulated wasteland.
