No you're right and I'm mistaken! I've just had a look on wikipedia: I do have a memory of seeing someone mention the delay to the fight putting it into the IBF's mandatory window and Glazkov being unwilling to delay his challenge. But seeing as I got the main detail wrong I may be wrong on that as well. Can;t see it mentioned in the wiki article anyway.
Like I said, either way, it's kind of irrelevant anyway. The IBF sanctioned the Klitschko-Fury fight, presumably knowing about the rematch clause, so they were basically lying to Fury. They never intended to recognize him as their champion. Unless they were looking for a bribe or something ......