This could only make sense in a place like Quebec.
By definition, someone who is a "federalist" supports the idea of Canada from sea to sea to sea. The BQ thinks that Quebec should separate. So as far as I can see if a "federalist" votes for the Bloq then they have switched sides and are now a separatist.
This is not just semantics - separation is a principle - but not one that federalists share. And I think it shows that Duceppe is really not thinking very clearly right now - probably too much coffee and not enough sleep.