Is there an explanation for the Hadith of Tayammum concerning Umar and Ammar, and did Umar contradict the explicit meaning of the Qur’an?

It was well-known that 'Umar ibn al-Khaṭṭāb and 'Abdullāh ibn Mas'ūd held the view that a person in a state of janābah (major ritual impurity) can only be purified by water, and is not permitted to pray by means of tayammum (dry ablution). Al-Bukhārī mentioned the story of the man who asked 'Umar because he was in a state of janābah and could not find water. 'Ammār ibn Yāsir then recounted the story of their performing tayammum during a journey, and that the Prophet, peace and blessings be upon him, taught them tayammum. Al-Nawawī explained 'Umar's saying to 'Ammār, "Fear Allah, O 'Ammār," as meaning: Fear Allah regarding what you narrate and verify it. As for 'Umar's apparent deviation from the explicit meaning of the Qur'an, he was exercising ijtihād (independent reasoning) and believed that a person in a state of janābah was not included in the meaning of the verse, and that "touching" (al-mulāmasah) referred to something less than sexual intercourse. He had not received the specific hadiths regarding this matter. There is nothing in his ijtihād that diminishes his stature, especially since the Prophet, peace and blessings be upon him, said: "If a judge exercises ijtihād and is correct, he will have two rewards; and if he exercises ijtihād and is mistaken, he will have one reward."