The author is not claiming to be a subject matter expert. The fact that they are a pediatrician is irrelevant. Their argument is well sourced, and "even" a pediatrician is able to read and understand papers in adjacent medical fields.
Dr Carroll is also known for the Healthcare Triage YouTube channel, which has numerous video explainers on topics like how to critically evaluate the strength of studies, the dangers of cherry-picking data, etc.
If it matters, my back pain largely went away when I switched to working while standing for three or four hours a day.
 E.g., http://www.sjweh.fi/show_abstract.php?abstract_id=3740
In fact, there was no difference in BMI reported between the standard condition and the intervention designed to decrease sitting time.
I don't know this literature much, but a separate analysis of the same study may show improved cardiometabolic risk profiles in workers who are encouraged to stand more often.
> Life years (LY) gained is a measure in health economics. It expresses the additional number of years of life that a person lives as a result of receiving a treatment.
And a 1% increase in LY after a 12 month intervention isn't a silver bullet, but it's nothing to sneeze at.