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.
The next day when the people entered the temple and saw their deities broken to pieces, they were shocked. They eventually discovered that it was the handiwork of a young man who had deviated from their ancestral religion and spoken against it. While demolishing the deities, Abraham had purposely spared the tallest one. When he was called by the people and enquiries were made of him, he pointed out that the big deity was still there, untouched and intact, and that people might put their questions to it. He said that if the big deity was god, then let it explain how the other deities had come to this pass. Although Abraham did not answer their questions directly, it was his indirect statement that sufficed to convey his message effectively. This shows that sometimes indirect speech is more effective than direct speech.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.