Ibn Umar knew well that the Messenger of Allah ﷺ performed Umrah three times, apart from the one he combined with the Farewell Hajj.
حدثنا النفيلي حدثنا زهير حدثنا أبو إسحق عن مجاهد قال سئل ابن عمر كم اعتمر رسول الله ﷺ فقال مرتين
لقد علم ابن عمر أن رسول الله ﷺ قد اعتمر ثلاثا سوى التي قرنها بحجة الوداع
[Its chain of transmission is daif] Ahmad narrated it: 2/70, and Nasai in Al-Kubra, no. 4218, from the hadith of Zuhayr, with this wording. Abu Ishaq narrated with "an" (without specifying direct hearing), and the basis of the hadith is agreed upon by Bukhari, Umrah, chapter, How many times did the Prophet perform Umrah, no. 1775, and Muslim, Hajj, chapter, Explanation of the number of the Prophet's ﷺ Umrahs and their times, no. 1255, from the hadith of Mujahid, with different wording than this.