Am I the only one who feels this is blatant and tasteless self promotion?
I think his central point - writing more helps you write better; StackOverflow is an easy and welcoming place to write more - is largely true, and I don't think he should be unable to say it just because it happens to promote his own interests.
I'm not sure why you doubt his sincerity. Stack Overflow is where he's spending most of his time these days, isn't it? Why wouldn't he be mentioning it in most of his posts? It seemed appropriate for this post. What would constitute a sincere blog post?
I don't think it's tasteless, but de gustibus non disputandum est, which Phillip K. Dick so eloquently translated as "Those people can have bad taste and I don't care."
Jeff Atwood says that being a great programmer is not about having technical skill, it's about communicating well. This is clearly an observation born out of narcissistic bias, since he freely admits that he is not a great programmer, and he is much more famous for writing words than code.