The 40% value was determined by Robert Lustig, I believe, as an approximation based on this study:

Prevalence of Optimal Metabolic Health in American Adults: National Health and Nutrition Examination Survey 2009–2016: https://doi.org/10.1089/met.2018.0105. (Open access).

The study is based on data from 2009 - 2016 and is a survey of 8,721 people. The researchers defined five factors for metabolic health and surveyed people who were not on medications. They found that only 12.2% of American adults are metabolically healthy. In addition, the researchers found that less than one-third of normal weight adults (defined by BMI) were metabolically healthy.

It isn't saying that you can be overweight or obese and not worry about metabolic health (although there is some research describing a small percentage of people who are overweight yet do not present metabolic syndrome), but it is suggesting that metabolic syndrome can appear in "normal weight" individuals and that obesity is not the only risk factor for developing chronic diseases such as dementia (although it does exacerbate it). It's the upstream problem of metabolic syndrome caused by chronically elevated insulin levels - related to diet.

There is also this study exploring normal-weight individuals who are metabolically unhealthy and risk factors for type 2 diabetes: https://doi.org/10.1016/j.metabol.2015.03.009. There's also many papers regarding the TOFI phenotype (thin-outside-fat-inside).

Also a nice open access review here on metabolic syndrome which includes some points I have raised here: https://doi.org/10.1007/s11906-018-0812-z

There is a wealth of studies linking western-style diets with all sorts of chronic diseases (including autoimmune disease). Just focusing on obesity alone doesn't cover the whole picture.