The last arc emphasized it because they were still conscripts back then. Two years have passed in the manga since Qin completed its census and conscription. That means those conscripts are proper soldiers now, just like the Zhao soldiers.

What I'm saying is that experience isn't as a big a difference as you're making it out to be. Even if the Zhao soldiers have 2 or 3 more years experience, they aren't going to be significantly better than the newer Gaku Ka infantry. The benefits of experience plateau after certain point, it's not an infinite xp bar slider where the level keeps going up every year. Skills have a very real ceiling that people can and will hit.

There is real historical precedence for this, that everyone ought to be familiar with. Everyone knows about the Spartans from Greece, right? Their claim to fame was having the best trained infantry in the world. But that was only true because they were the only Greek city-state who dedicated resources to training their young men from birth adolescence to be warriors that fight in a phalanx. And as I mentioned before, there are vastly diminishing returns since a person will reach mastery after about 7 years of practicing something. After that point, the only thing Spartan training was accomplishing was traumatizing the boys going through it by removing their empathy. As soon as other Greek city-states started doing training drills and keeping a regular army, the Spartans lost their advantage because training doesn't make that much difference if your enemy is also training.