If the book lets Space Wolves be Librarians with only a passing reference to "Rune Priests," and lets them be Techmarines with only a passing reference to "Iron Priests," why is the same hand-wave not made to let them be Apothecaries and just called "Wolf Priests" for roleplay purposes? Is a Wolf Priest that much more different from an Apoc than a Rune Priest is from a Librarian, or a Techmarine from an Iron Priest?
I suspect we'll see advancement schemes for all three of the above SW-specific specializations in a supplement eventually, but for the time being, why not?