All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
First of all, you should know that these people do not cease to search for ways to undermine the Quran, as they believe that the Quran is distorted, and they accuse those who transmitted it to us and reported the Sunnah of the Prophet to us of Kufr (disbelief).
The purpose of their mention of this narration from Ibn ‘Abbaas or other narrations by the Companions is to confuse the laymen of Ahlus-Sunnah; this misconception may affect some people because of their lack of knowledge and, as a result, they fall into this trap. Therefore, one must avoid accessing their websites unless one has profound knowledge (in order to not be affected by their misconceptions). Ash-Shaatibi said in Al-I’tisaam, “Al-Baaji narrated, 'Maalik said that it used to be said, 'Do not lend your ear to someone whose heart is astray, as you do not know which misconception he would spell into it.''”
Secondly, Allah promised to protect this Quran from any distortion or alteration; Allah says (what means): {Falsehood cannot approach it from before it or from behind it; [it is] a revelation from a [Lord Who is] Wise and Praiseworthy.} [Quran 41:42]
Al-Baghawi reported in his Tafseer (exegesis), quoting Az-Zajjaaj, “It means that this Book is preserved from any addition or deletion...”
Thirdly, what you quoted from Ibn Hajar that he authenticated the narrators of this hadeeth does not necessarily mean that this hadeeth is authentic, as there may be, for example, an interruption between one narrator and another and he did not hear it from him, so the hadeeth or the statement of the Companion would be weak due to the interruption. For example Al-Haythami said in Majma’ Az-Zawaa’id, while commenting on one of the ahaadeeth, “The rest of its narrators are trustworthy, but Khaalid ibn Ma'daan has never heard from Mu'aath.”
Also, As-Suyooti reported in Al-Itqaan from Ibn Al-Anbaari that he held the opinion that this narration from Ibn 'Abbaas is weak, and he opposed it with another narration.
Fourthly, even if we were to presume that this statement (of Ibn 'Abbaas) was authentic, it is impossible that there was a mistake at this place from the scribe and that it remained written as such in the Mus-haf (the written text of the Quran) and that the Muslim nation transmitted it generation after generation from the era of the Prophet until the present day. Al-Qurtubi said in his Tafseer, while clarifying a context similar to this one which was said to be a mistake from the scribe:
“If this were a mistake (a word that is written wrongly), then those who taught the Quran from among the Companions of the Prophet to their students from the Muslims would not have taught it with that mistake; rather, they would have corrected it with their tongue, and they would have taught the Muslims in a correct manner, and the fact that the Muslims have transmitted this orally in an identical form to that which is written is a clear-cut evidence that this is correct and has no errors and that this was not a mistake by the scribe...”
Ibn Taymiyyah reported the same thing.
Fifthly, even if we also were to presume that that statement was authentic, some scholars interpreted that the saying of Ibn 'Abbaas meant the mistake in making the choice and what is more appropriate of the seven letters (ways of reading) in which the Quran was revealed) to gather the people around it, and not that what is written is an error that does not belong in the Quran. This was also reported by As-Suyooti . For more benefit, please refer to fatwa 82974.
Allah knows best.