So the book invokes individuals with low levels of psychopathology and negative thought. They have very high scores in psychometric health tests.
Yes most of us have average symptom levels of mental disorder, and there are those with very high and very low levels of mental disorder.
This refers to health strategies:
Psychology used to be associated mostly with illness, maladaptive practices and negative thoughts, so in 1998 the term positive psychology became popular.
Mental health can be defined by positive or negative outlook, self-belief, judgement, good sleep, efficient organization, mental energy.
Negative views of the past, of oneself, of others, of society, sometimes nightmares, bad daydreams, and poor logical processing.
It is possible for someone in very good mental health to have very few of those types of negative thoughts.
Positive psychology and the absence of negative thoughts can be measured using various psychometric instruments and assessments:
Positive and Negative Affect Schedule (PANAS): A questionnaire measures both positive and negative affect, capturing the presence of positive emotions (e.g., joy, enthusiasm) and the absence of negative emotions (e.g., fear, sadness).
Satisfaction with Life Scale (SWLS): This scale assesses an individual's overall life satisfaction, reflecting their cognitive evaluation of their life as a whole.
Flourishing Scale (FS)
Resilience Scale (RS)
Self-Esteem Scale
Positive Psychotherapy Inventory (PPTI)
I have heard people described as being "almost completely devoid of complexes", i.e. they are very rare and fortunate to have a can do attitude about everything, social confidence and no particular secrets or conflicts with themselves.
It would be cool to be the human on the left. To clarify what you questioned: Those who are especially well-balanced may not be very wise or performant in other psychometric scores for knowledge or life skills, for example, it just means that they sleep well, wake up feeling good and ready for the day, don't worry much, enjoy their lifestyle, feel socially confident and optimistic to achieve what they want, they enjoy a lot of things in life, have few subjects that they find embarassing or complexing, deal with challenges well, have few bad memories or social hangups, are very satisfied with themselves, and enjoy themselves... The other end of the scale is often people with trauma, they have mood-swings, depression, possibly agression or argumentative cycles, out of control thoughts patterns which torment them, negative views of society, family, contemplate suicide, often dwelling on memories, have nightmares at night, bad sleep, strong ego, judgement... Most of use are somewhere in between those two extremes. The scientific way to measure it is psychometric tests.