To be clear, the entire quote is:
A frequently asked question is: Why doesn't Avalon just use SVG? On the face of it, it seems strange to invent a new way of representing vector graphics in markup when a standard already exists. However, the principal advantage of these shape classes is that they have been designed to integrate into the Avalon programming model. The vector drawing elements derive from the same FrameworkElement base class as all other XAML elements, and follow the same naming conventions. SVG has its own set of conventions for element and attribute names that is at odds with the existing .NET Framework class library. Furthermore, SVG elements were not designed to fit into the Avalon object model. By not using SVG, Avalon ensures that vector graphics can be mixed in with any other kind of markup in your XAML, and can take advantage of all of the same layout facilities. (Note that in the version of the Longhorn SDK documentation released at the 2003 PDC, the XAML elements used for vector drawing are sometimes referred to collectively as WVG (Windows Vector Graphics). However, Microsoft is no longer using this name, because it implies, incorrectly, that these elements are somehow distinct from all of the other elements supported by Avalon.)
Which is a lot more than just "SVG has its own set of conventions for element and attribute names that is at odds with the existing .NET Framework class library.".