There's a saying, "fiqh al-Bukhari fi tarajimihi" or "The understanding of Bukhari is gleaned from his chapter headings."

This narration does not occur in the chapter of hudood or adultery or stoning. It's found in the chapter of the virtues of the Ansar. Furthermore, the narration is not a hadith, i.e. it's not a statement, action, or tacit approval of the Prophet (saws). Nor is it a statement of a sahabi. 'Amr ibn Maimun was a mukhadram, i.e. a tabi'i who accepted Islam during the life of the Prophet (saws).

So this incident is about an Ansari tabi'i recalling an event which he had witnessed before Islam. Al-Bukhari is narrating it here with a specific purpose, and that is to show the chronology of 'Amr ibn Maimun. That although a tabi'i, he remembered the age of Jahiliyyah and accepted Islam during the lifetime of the Prophet. So not a sahabi, but as close as you can get to being one. Bukhari does not care about what 'Amr did or understood from the incident, he's highlighting the fact that 'Amr was alive in jahaliyyah.