How do you see it as derogatory? I'm a native English speaker and have never thought of it that way. I didn't click the link, but did appreciate his short summary – and upvoted him for it. ;)
That clearly wasn't the case here, so I was wondering why the author choose to use it. I realize that meaning has changed over time, but I was wondering what meaning he (and others) intend when it is used.
Example:
poster1: only retards use vi, notepad rules
poster2: huge list of reasons why vi is better than notepad
poster1: lol tldr
We already have terms like "summary", "digest", and even "précis"; why create a new term imbued with snark?
But not always. You have to consider how it was used to tell whether it was meant with ill will, the term tl;dr on its own doesn't necessarily tell you enough.