Sorry for the delay. Here is the מראה מקןם. It's a רא"ש in ערבי פסחים סימן מHe didn't discuss the question explicitly, and he may may not apply his answer to this question. Though, based on this, I'd have a hard time calling this כפירה.
If you believe Hashem couldn't make a miracle that all the shevatim ended in a round number, that's obviously kefirah. But the question is, to what end would Hashem have made such a miracle