The Wikipedia article is fine. The other one's wrong in parts. Remember that anyone can post a blog.
'Dislike' is not a negative in syntactical terms. "I don't dislike her" does
not mean "I like her", as I've explained.
In practical terms, if you don't disagree, you
either agree,
OR you don't care enough to make a decision.
If you treat all English words starting with 'dis' as negatives, then if you're not diseased, you're eased - which is silly.
There's probably a spectrum of such words, and on one end "not dis***ed" could mean "***ed".
This applies to all such negating prefixes - un, in, anti, etc. If I'm not anti-war, that does not make me pro-war. You can't claim that because I am not at one end of a spectrum, I must be at the other end. That is black and white thinking, and it's invalid.
You need to think about the logical meaning of words. Did you read the Wikipedia article?
"For instance, "I do not disagree" could mean "I certainly agree". Further statements may be necessary to resolve which particular meaning was intended.
Because of this ambiguity, double negatives are frequently employed when making back-handed compliments. The phrase "Mr. Jones was not incompetent." will seldom mean "Mr. Jones was very competent" since the speaker would have found a more flattering way to say so. Instead, some kind of problem is implied, though Mr. Jones possesses basic competence at his tasks."
http://en.wikipedia.org/wiki/Double_negative