The way I see it, Captchas are an approach to fend off _untargeted_ spam bots. If someone specifically targets your website and tailors his spam bot accordingly, then you have a problem, but that is not inherent to Slider Captchas.

On the other hand, a Slider is much more user friendly than a scrambled image or having to select all pictures with light bulbs.

So I think this article misses the point a bit...