The Second German Empire was not "absolutistic." The Kaiser had the hold on foreign policy (like the US prez), and could choose his chancellor (like the US prez chooses his secretaries) but the Reichstag had important powers as well, such as finance, the basis of all constitutional government. This would not affect the use of the word "Reich" anyway. In German, it does not only mean "Empire" in the Roman Law tradition, it also has the connotation of "subject to home law."
Honny soit qui mal y pense!