It very much ignores the underpinning lack of early help resources to meet need, and that this is the key aspect. This bit is particularly baffling given that it actually says that it wasn’t the role of SWIS to provide early help, just providing statutory services in a different office (the school):

Social workers who are part of SWIS were physically located in schools and had an allocated caseload
of children who have been referred to CSC and met the local authority’s threshold for a service
(ranging from those who are children in need to those who are children looked after). Whilst the main
focus of SWIS was statutory work, there may have been aspects of the role that could be described
as preventative, such as advising staff and families when they have concerns that they would like to
discuss, however SWIS did not include Early Intervention work which continued to be provided as a
separate service.

If you are testing a new vaccine, then you can find out whether it works better than no vaccine or better than another vaccine or differently for men and women, for black people and white people, for children and adults, etc. because each dose of the vaccine is chemically and medically the same. Because the ‘input’ is the same, you can measure differences in outcomes and be reasonably confident they were caused by the input. If, however, you attempted to report an RCT of vaccine, and claimed that it didn’t work, while also admitting “of course, the actual contents of the vaccine were different in different places”, you’d never get published and would be laughed out of the lab.

Running this expensive trial tells us nothing about whether putting Social Workers in Schools “works” because there is no such (simple) intervention to be tested.