SAHM is something that didn’t happen before the 1940s. Before that the women worked with their baby in a sling or the kids went off to their jobs.
I often see this kind of thing stated on MN. It doesn't reflect my family's experience at all. My grandmothers were both born in the 1900s. My maternal GM was from a very poor family living in a rural area. Her father, who had had a manual job, died while she was still at school. After that her mother had to become the breadwinner, but prior to that she was at home looking after her very large family and keeping house. She may have been looking after chickens and growing vegetables as well, but she certainly wasn't in paid employment.
My GM left school at the earliest age possible and worked at whatever jobs she could get until she married (maid of all work, shop assistant), and then she was a housewife and mother. She did have a job during WW2 when she took over an insurance round from a man who'd been called up, but when the war ended she gave that up and never had a job again, although I think she may occasionally have helped out when my GF's employer wanted extra cleaning or kitchen work done. My GF was a gardener, so they were very far from affluent.
Her experience was absolutely typical for that side of the family. None of my great-aunts had jobs once they had young children. The closest any of them got to earning their own money was taking in summer visitors. Back then they usually had a shack near their (very small) house and in the summer the family moved into the shack and the visitors rented the house, with the women of the family doing the cleaning and providing meals.
My other GM came from a lower middle class family and worked in an office when she left school but again gave up when she married. She left her abusive husband while her children were still school age and fortunately had the support of her own family so was able to get another full-time office job. Eventually she gave up work so she could look after my cousins while my aunt worked. Her sisters also did not work outside the home after marriage, at least while their children were young. None of them had what you'd call a career nowadays. (Criminal waste of talent, they were a bright lot, as were my great-aunts and GM on my Mum's side.)
Maybe the reference is to working class women living in cities very close to extended family who could look after the children while they worked in factories or mills or shops. That's only a small proportion of UK women before WW2, though.