Why can't the Corded Ware Culture be a part of the Kurgan hypothesis of the PIE Urheimat? These two things overlap in time and place. Gambutas included the Corded Ware culture in her theory. I'm not sure if there is a "Kurgan Culture" or if the Corded Ware Culture built Kurgans.
...the Corded Ware culture was once presumed to be the Urheimat of the Proto-Indo-Europeans... Today this idea has lost currency, as the Kurgan hypothesis is currently the most widely accepted proposal to explain the origins and spread of the Indo-European languages. Corded Ware Culture- Wikipedia.com
Edit: I think that this "Kurgan Hypothesis" may be refering to the spread of later Tumulus Culture, with confusion arising from the interchanging of those words. This Middle Bronze Age expansion is my understanding of how IE languages spread outside the steppe.