Both articles are in Joel's voice and credited to Joel. You can't plagiarize yourself. I also used the word "might".

The Founders at Work interview is at http://www.foundersatwork.com/joel-spolksy.html . The Inc article is not a copy and paste, but it triggered the "seen this before" meter.

Plagiarizing himself? Huh? Is that even possible?

Yes, and it's a big fucking deal in academia -- you're bloating your publications / citations / reputation without actually contributing anything.

http://en.wikipedia.org/wiki/Plagiarism#Self-plagiarism

From your linked article:

"Typically, self-plagiarism is only considered to be a serious ethical issue in settings where a publication is asserted to consist of new material, such as in academic publishing or educational assignments [13]. It does not apply (except in the legal sense) to public-interest texts, such as social, professional, and cultural opinions usually published in newspapers and magazines."