It's obvious that he is expressing an opinion. Those who disagree with him can express an opposing opinion. Just because someone says something, doesn't mean that they need to back it up in any scientific way, especially in a community based around discussing ideas and sharing opinions.
Making such statements without providing any evidence for them is not only intellectually vacuous (since if there is no need to provide evidence, then any claim can be made), but it leads to bias in others where people assume that because an argument was not made, that the information is "well known" or the person providing the information is an expert.
There's nothing wrong with sharing opinion. I'm guessing that the reason he came to the conclusions he did was based on personal experience (probably mixed with some reading); if so, then he shouldn't have said "the reasons why human-beings procrastinate is to feel in control of their life", he should've said "I noticed that I seem[ed] to procrastinate not because of (...), but because I didn't feel in control of my life". If his evidence wasn't introspective, then he should have shared whatever his evidence was. If he didn't have any evidence other than plausibility, then he should've framed what he was saying as a hypothesis (and ideally still explained his reasoning).
You don't have to back up everything you say with scientific evidence, but you shouldn't make sweeping claims, especially in fields like this one where the jury is still out, without either providing evidence or qualifying your claim. (And I don't buy the "it's too much extra writing" argument; he could've inserted "I have a hypothesis:" after "Dear procrastinator" and had it completely covered.)
And no, I'm not just being nitpicky; a number of well-known biases like the primacy effect, confirmation bias, the "trusting the confident statement" bias I mentioned earlier (which I can't remember the name of for the life of me), and wishful thinking (in this case, wishing for a solution to procrastination) mean that humans are very vulnerable to forming irrational beliefs when ideas are presented in this way. An ounce of prevention, in the form of stating the reason you believe what you believe at least when you're making new or potentially controversial claims, is surely not too much to ask given how easy it is and how far it goes.
(Perhaps "[citation needed]" came across as snarky? Next time I'll write "What's your evidence?".)
The rest of my posts weren't complaining, either; mainly, I was arguing for the importance of getting people to do 1 and 2 above, and against the notion that it wouldn't be better for opinions (of the "claims you believe to be true" variety, not preferences) to be either supported or marked as hypotheses when they are expressed (at least in this sort of context).
I was definitely not dissenting; my criticism would be exactly the same even if I agreed with the expressed claims 100%.