According to the following sources the definition is more limited to the ventricles
Amboss states the defintion is "Left or biventricular dilatation with structural and/or functional systolic dysfunction in the absence of coronary artery disease, abnormal loading pressures (e.g., valvular heart disease, hypertension), or congenital heart disease"
https://next.amboss.com/us/article/nr073h?q=dilated%20cardiomyopathy#pNWLXN0
Uptodate states "A diagnosis of dilated cardiomyopathy requires evidence of dilation and impaired contraction of the left ventricle or both ventricles"
https://www.uptodate.com/contents/causes-of-dilated-cardiomyopathy?search=dilated%20cardiomyopathy&source=search_result&selectedTitle=1%7E150&usage_type=default&display_rank=1#H2
Source: AMBOSS - https://next.amboss.com/us/article/nr073h?q=dilated%20cardiomyopathy#pNWLXN0