Ibn Abbaas said that the abrogation is the other way around, 4:93 abrogated 25:70 because the former was revealed in Medina while the latter is Meccan.
Al-Khazraji, in his book نفس الصباح في غريب القرآن وناسخه ومنسوخه, volume 1, page 291, said that Zayd ibn Thaabit, may God have been pleased with him, said that 25:70 was revealed six months before 4:93. He ought to know, he inscribed both by dictate from the Prophet (PBUH) when they were revealed.
Ibn Abbaas is reported to have given a fatwa of no repentance to a man whom he knew wanted to kill a believer, so that he wouldn't! But his opinion, official fatwa, is that repentance is accepted. This contradicts his assertion above that 4:93 is the final verdict. It also casts doubt on the reports of his fatwa.
Al-Khazraji, in the same book, volume 1, page 291, quotes An-Nahhaas saying that Ibn Abbaas had three opinions: that 4:93 was not abrogated, that it abrogated 25:70 and that 25:70 abrogated it! I doubt very much that a reputable scholar like Ibn Abbaas, may God have been pleased with him, would vacillate like that. Odds are the narrations are weak or fabricated.
On pages 546-547, volume 2, of the same book, Al-Khazraji mentions that Makki mentioned that some people rejected the claim on the basis that 4:93 is a declarative statement.