وَيْلٌ يَوْمَئِذٍ لِّلْمُكَذِّبِينَ
And its position sums up the whole structure of the sūra: the refrain comes after a diagnosis of a single verse. The sūra had begun with a section of seven verses; it ends with a section of one.
One further observation: between the last refrain and the sūra's last verse there is no section at all. That is, the refrain stands immediately before the closing sentence — the last judgement before the sūra's last word.