Well, there’s two things going on here. Yes, the Bakkula Sutta is, on the whole, probably a late text. But the interpolated sections, giving the response of the redactors, are clearly later still.
To be clear, when we refer to the “commentary” we are usually referring to the primary strata of commentarial literature primarily by Buddhaghosa and known as aṭṭhakathā. These were composed, or rather translated and edited, in the 5th & 6th century in Sri Lanka.
The interpolated choruses in the Bakkula Sutta are not from there. The commentaries refer to the interpolations, showing they must have pre-dated them. The commentary says that the interpolations happened at the Second Council, which seems reasonable.