Selkirk is the parent company who manufactures SuperVent chimney. Since Selkirk also produces several other brands, SuperVent wouldn't be compatible with any other chimney system they make outside of SuperPro.
Does the 6 inch selkirk anchor plate directly accept a 6 inch section of class A insulated flue?
A 6 inch Selkirk-branded anchor plate (SuperPro or SuperVent etc.) would indeed connect directly to a section of whichever brand of Selkirk 6 inch class A insulated double wall chimney you are installing. That being said, you cannot connect a Selkirk-branded anchor plate to a Simpson DuraVent product, for example, and would be limited to whichever brand of Selkirk chimney the anchor plate was made for use with.