I am surprised, given your expertise, that your article thesis is so misleading.

One of the major facets of the study of birth order is that its etiology is exactly what you described as a throw away concept at the end of your article.

Birth order characteristics do hold true in many cases, compounded by the unique variables of each family, eg ages, deaths, financial shifts, demographics, numbers of years between siblings, etc. Gender is a major variable and determinant.

People who study birth order know that a prime reason these generic characteristics and stereotypes develop at all is due to the "projection" of the parents or key caregivers, which of course involves their own birth order, childhood experiences, unconscious and unmet needs, etc.

Birth order absolutely does matter and the more people understand that, the more they can then start taking back projections and attempt to parent through the lens of the child who is in front of them.