That's a valid point, and I don't think that, even if Wolf is planned, that he was ever intended to be released before the ones we just got. All of the pre-ballot content was likely planned soon after they saw the numbers for Mewtwo, if not before. Obviously, earlier packs would be prioritized by the developers, so maybe they simply hadn't made it to that content yet.
In any case, if we're on the upper end of the generally expected two to four or five characters, I think it's reasonable to say that they have already been working on it for release relatively soon, within the next 2-3 months. As noted by many before, the N64 stages shown in the "New Content Approaching" video looked mostly complete. I don't expect many, if any, DLC characters, pre- or post-ballot, much farther in the future than December or January at the very latest.
I said it specifically about Wolf before, but it could apply to any characters currently being worked on: they likely started development of the amiibo shortly after completing the model for the character. Now, I know during game development, they don't always complete a full model before programming functionality, but this is also add-on content for already completed software, and Sakurai/Nintendo/development staff are aware of the demand for amiibo. It's very possible that that portion of the process is being expedited or developed alongside everything else in order to get it out for purchase at a faster rate for the customer base.