Responding to this question is a bit complicated as it requires a degree of inferring the intent of the original author of the response to the article. Based upon this passage from the author:
Children are not ready for traditional team sports until perhaps the
last year of primary school at best. What he is pleased to call an
"all must have prizes" culture is experienced professionals ensuring
that all primary children develop confidence in their physical
identity, each optimally developing individual physical skills and
coordination.
In order to ensure that definitions do not play too much of a role, I'm going to assume that primary school when a child is approximately 10 years old, traditional team sports are sports that have organized rules that require multiple players per team (e.g. baseball, kickball, volleyball, etc.), and that competitive team sports are when fixed teams compete against each other in some sort of league play to prove they are better than all of the other teams.
With these definitions in mind, to address the first point, that children are not ready for traditional team sports and that they have a negative impact on child development, the answer is an emphatic no. The reason for this is many fold: first, children naturally develop an interest in team sports around 5-6 years old and this is seen as a developmental milestone. Next, studies have been conducted that have show an positive correlation between team sport participation and physical and mental development which is also born out by studies into bone density development in children. Finally, participation in team sports has been shown to lead to positive self-esteem and that structured activities such as sports have a positive impact on mental development.
With regards to the second point, that competitive team sports have a negative impact upon mental development, the answer is yes this does seem to be the case; however, a fair bit of point more towards burnout and injuries as the drawbacks as opposed to mental development. That said, right now it appears that the trending recommendation is that children are best kept from competitive team sports until around age 10, when they fully understand effort and ability as it relates to winning. This appears to be backed by studies that have shown that failure in competition backed activities can have a demotivating effect versus mastery based activities.1
Thus to summarize, sports play an important role in the development of children and while there is some evidence to show that competitive sports can have a negative impact on child development2 the consensus is almost universally in favor of encouraging children to participate in sports from a young age and then having them grow into more competitive league play as they get older.
- Note that a fair degree of what I found with regards to the negative effects of competitive sports in young children was based around injury and the extent of training necessary to be competitive from a young age. I had a hard time finding papers that discussed long term negative effects that weren't due to the potential for injury or due to time invested in training for sports.
- A mild editorial, but based upon the papers I have read and skimmed, it's hard to say what influence parenting might have upon a child with regards to competitive sports. Given that most papers agree that children naturally want to participate in sports among themselves, they should be familiar with the concepts of competition and loss before the recommended age of 10 cited in one of the previous articles.