I think there are legitimate grounds for Catholic Knight's definition, but the problem with it is that one of the arguments contra the Sedevacantists is that there would be popes in perpetuity. While Sedes have argued against that in that we have had antipopes, I think the response to that is, we have always had a legitimate pope at the same time.
Now, the Bennys are confronted with a false pope on the chair, no legitimate pope reigning, and are confronted with break in the perpetuity of legitimate pontiffs absent the periods when there is no pontiff, false or true - the normal sedevancy upon the death of a true pope.
The gun of the perpetuity argument now points at them, no?