That, and #17's gains don't have to be as YUGE as people seem to take it at face value. Remember that SSB is all about Ki control and regulation, at least anime-wise, hence why Goku has been using it in order to evaluate most of his recent opponents. There's honestly nothing wrong with placing #17 at current SS2 Goku's level if you go by this theory.
Hell, even if taking things at face value and saying #17 ~ SSB Goku, it's honestly not that ridiculous at this point. Not only did we have Freeza surpassing SSB with 4 months of training, but also had Piccolo/Gohan at least getting up to Post-God Base Goku's level in what seems to be a couple of weeks or months. Considering #17 has likely been training in most of his spare time since the Cell Games going by what Dende suggested, it honestly isn't that crazy at this point.