Quoting the article:
The filters all operate on pixel data, not SVG maths. As such, they're independant of Cairo, Libart, or whatever one wants to draw with.
But Cairo does operate on pixels, AFAIK. It renders vector data into pixels. So it makes a lot of sense to implement e.g. blurring directly in Cairo, not in Inkscape on bitmaps returned by Cairo.
Still, as I wrote, I can do quick-and-dirty blurring implementation even in our current renderer, once we have SVG support for filters.
it uses libart to draw
No, we don't use libart.
Sure, it might not be able to (currently) draw 10 gazillion polygons per second like Xara claims to do, but does Inkscape really need that level of performance?
Oh yes, we surely need as much performance as Xara has, and ideally more :)
-- bulia byak Inkscape. Draw Freely. http://www.inkscape.org