You‘re correct, the Corded Ware population was dominated by one single lineage: R1a-M417. With that being said, they weren't 100% R1a-M417. The Bohemia ancient DNA paper revealed that R1b-L151 is the most common Y-DNA among early CW men and one branch ancestral to R1b-P312, the dominant Y-lineage in Bell Beaker. So, the spread of hg. R1b-L151 (including the subclade R-U106) is now certainly associated with the expansion of the early Corded Ware people. Therefore, as suggested in the study, Bell Beakers probably formed to the west of Bohemia, closer to the Rhine, and spread from there in all directions ( into France, Italy, and Iberia, and northwest into Britain and Ireland).
So, Bell Beaker was just a development from Single Grave Corded Ware. Furthermore, Proto-Italics partly descended from these Central European BB. That‘s why the Etruscans and Latins were majority R1b, specifically R1b-L2 and R1b-P312, which were of Bell Beaker and Corded Ware origin.