The words of Sayyidna Ibrahim (علیہ السلام) were not untrue, but only allusions
قَالَ بَلْ فَعَلَهُ كَبِيرُهُمْ هَـٰذَا فَاسْأَلُوهُمْ إِن كَانُوا يَنطِقُونَ (He said, "Rather, this is done by this chief of theirs. So, ask them if they were to speak." - 21:63) When the community of Sayyidna Ibrahim (علیہ السلام) got hold of him and asked him to confess his guilt, he said "The big one among them has done it. Why don't you ask him, if he can speak"?
The point to consider here is that the idols were broken by Sayyidna Ibrahim (علیہ السلام) and its denial and attributing the deed to the big idol, was contrary to fact and constituted an untruth. The exalted position of Sayyidna Ibrahim (علیہ السلام) renders such a conclusion untenable. The exegesists have put forward several explanations to sort out this matter. One of them is that his statement is in the nature of an assumption i.e. why not considering the possibility that the deed was done by the big idol? And a supposition which is contrary to facts is not a lie as the Qur’ an itself says إِن كَانَ لِلرَّحْمَـٰنِ وَلَدٌ فَأَنَا أَوَّلُ الْعَابِدِينَ (If the Ralhman [ All-Merciful ] has a son then I am the first to serve him - 43:81). But the most convincing and straight forward explanation is the one adopted by al-Bahr ul-Muhit, Qurtubi and Ruh ul-Ma` ani, which says that it is a metaphoric attribution (اَسنَاد مجَازی), that is the act which was done by Sayyidna Ibrahim (علیہ السلام) was attributed to the big idol by way of metaphoric attribution because it was this idol which, by reason of the reverence that it commanded in the whole community, persuaded Sayyidna Ibrahim (علیہ السلام) to do what he did with the smaller idols. It is like amputating a thief’ s hand and then telling him that none but his own misdeeds are responsible for the amputation.
On hearing such replies from Abraham, the people could have accused him of insolence and taken him to task, as usually happens in such cases. But, in spite of their being polytheists, there seemed to be some sense of reason left in them. So, realizing the force of the rationality in his replies, they felt ashamed and admitted that they were wrong. Had they not subsequently fallen a prey to prejudices, this experience would have been sufficient to lead them towards the true faith.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.