We know that khum is 140 kms far from makkah. So it means only those are present during announcement of khilafat of imam ali who are people of Madinah not makkah and yemen.
Is not it better that Rasool announce it in Arafat so that all people will be present?
Every word that is against the verses of quran cannot be accepted by a mindful muslim even if someone tries his best to twist its meaning for his own sectarian intentions.
The words of Imam Ali (عليه السلام) are clearly explaining the words for clarification of the term majority and keeping the unity in nation.
The sunni takes their favorite term of Jamah (جماعۃ) as describe the sermon for majority. Imam Ali peace be upon him gave us a thorough definition for the term Jammah. Al Mu