that means, when the kafir does tawbah in this world - it is accepted here in this world (sparing him death-penalty), and this (tawba from this world) will also help him in the hereafter.
it is among preliminaries that tawbah, repentance or remorse done after death or in there hereafter is of no use. the qur'an tells us of kafirs who will repent on judgement day and ask for one more chance, which will be rejected.
this clause is specifically mentioned in ashbah, because some maliki jurists (such as qaDi iyaD) say that the tawbah of a blasphemer is inadmissible near Allah ta'ala [ghamz uyun al-baSayin sh. al-ashbah wa'n nazayir, 2/190 ]
Allah ta'ala knows best.
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in sha'Allah, a note will be added in the next edition; the book is still being reviewed.