It was. The Germans slaughtered far more than the Japanese did. Mengele also performed the same kind of experiments carried out by Unit 731. There is no question Unit 731 existed and did carry out some rather brutal experiments. The problem with some of the accounts is that they are one-sidedly Chinese. The Chinese, like the Japanese, are not known for being dispassionately factual in their accounts of the war. (For example, China asserts that it, and not the United States, defeated Japan during WWII). The estimate of 400,000 dead from chemical and biological bombings is yet another airy Chinese estimate that can't easily be verified. What we know about the number of people killed by the Germans is well-documented. The body count amassed by the Japanese in China is a little more dubious - the numbers have been put together by a regime that insists that American forces used chemical weapons during the Korean War. Even if you take their numbers at face value, they don't approach the scale of German war atrocities.
As the article points out, you also take into account quality, not only quantity. Both were huge war crimes, and a blot on human civilization.