Although the CSCW paper you reference (Schmidt & Bannon, 2013) does not seem to mention the historical relation between HCI and CSCW explicitely, it does reference an early CHI paper on the significance of CSCW by Bannon (1992): Perspectives on CSCW: From HCI and CMC to CSCW.
In chapter two of this paper ("Issues in HCI") problems within HCI are discussed which have lead to the emergence of the CSCW field.
In this Section, I concentrate on a particular area of research,
human-computer interaction (HCI) and argue that the field has
been encountering a series of crises concerning the utility of its
conceptual approach and empirical methodology over the last few
years1. Later I will argue that the shift in
perspective underlying CSCW research can be seen to avoid or
reduce the severity of some of these problems. While some argue
that CSCW is simply an "extension" of existing HCI research and
development, my point is that one can detect a switch not
simply in emphasis, but also in conceptual frameworks, and even
methodologies, in the new research field.
My basic thesis is that this mainstream "cognitive science"-inspired HCI research has come up against a number of problems, both in terms of its research agenda and the usability and utility of its empirical results for the software design community (Bannon, 1990, 1991)
Concluding from this I thus feel it can be said that CSCW split off from HCI when it first originated due to dissatisfaction with the major methods employed at that time.
Schmidt, K., & Bannon, L. (2013). Constructing CSCW: The first quarter century. Computer Supported Cooperative Work (CSCW), 22(4-6), 345-372.
Bannon, Liam J. (1992). Perspectives on CSCW: From HCI and CMC to CSCW, In EW-HCI’92:
Proceedings of the International Conference on Human-Computer Interaction, August 1992, St.
Petersburg, Russia, pp. 148–158.