How could a Companion pray without reciting Surat Al-Fatiha, when the Prophet (peace be upon him) commanded its recitation after prayer?

The Hadith narrated by Bukhari from Abi Sa'id ibn Al-Mu'alla, in which the Prophet, peace and blessings be upon him, mentioned that Surat Al-Fatihah is the greatest of surahs, intends to inform him of its immense status and position in the Qur'an, and not to teach him its letters and words. This is supported by the fact that Ubayy ibn Ka'b, in a similar hadith, already knew Al-Fatihah by heart and recited it in prayer when the Prophet, peace and blessings be upon him, informed him that it was the greatest surah.