In this case, a systematic error wouldn't change the conclusion as long as the same method was used for all countries. As for the broader argument about people

In this case, a systematic error wouldn't change the conclusion as long as the same method was used for all countries. As for the broader argument about people dying with COVID vs people dying of COVID, the excess mortality rates are consistent with the assumption that the reported death rate from COVID in Western nations is pretty accurate, and may even undercount the true figure (see e.g. https://www.nature.com/articles/d41586-022-00104-8 https://www.nature.com/articles/d41586-022-00104-8).