And certainly simpler systems work for K-3 because kids are not cohesively learning performance-based material at that young age.
At these ages, many kids might not yet be exhibiting signs or symptoms of learning disorders or spectrum behaviors; but it is even more common that littler guys are not easily differentiating abilities yet. For example, first graders might read at different speeds, but they are all reading; does it make sense to give one kid an A and another a B? Not really. Hence the Czars comment.
But nowhere in the original explanations against which the Czar was railing is this program limited to K-3. Rather, this outrage most certainly applies to grades 4-8 as well. While letter grades are new to the Цесаревич, for example (straight As, cough cough), he is evidently adjusting to them well.
But an 8th grader who cannot read, do multiplication, measure a two-inch line when given a ruler, and so on, is allowed to graduate with a BA (basic abilities) grade? Something here is so bad it reeks all the way to Muscovy. And the real reason, it seems we all agree, is to help hide deficiencies in school districts.