Haha, this is hilarious.
So, they can't use the Ghadir narration to clearly confirm beyond the doubt that the Prophet (SAW) was referring to successorship instead they go fishing for other narrations - typical.
Where they fail miserably is also disapproving the other plethora of narrations that strongly indicate if not more or less suggests who the first Caliph should've been, and was. Namely, Abu Bakr (RA).
I can't comment on the authenticity of the narration, I'll leave that for the experts, but here are my thoughts:
1) This narration is when Ali (RA) was left beyond during the Battle of Tabuk that took place in month of Rajab in the 9th year after the hijrah. The Prophet (SAW) passed away during the 11th year after the hijrah. So, how could the Prophet (SAW) mean to say here that he's the undoubted and exclusive individual to succeed him? Why would he leave his deputy behind if that's what the Prophet (SAW) genuinely meant. Why would he (SAW) be thinking of who is going to succeed him when Deen itself is not complete?
2) The Prophet (SAW) confirmed in many narrations that Caliph would come Quraish after his (SAW) departure. How come the Prophet (SAW) didn't mention a name?
3) The narration cannot be reconciled with the Hadith of Thaqalayn or Ghadir because here the Prophet (SAW) is not addressing an audience, but at Ghadir he was because just after it had reached his attention that some of the new Muslims were giving Ali (RA) a rough time the Prophet (SAW) wanted to make an historical yet infamous statement to be clear to everyone of Ali's (RA) position in this Ummah. In this narration the Prophet (SAW) is expressing his closeness to Ali (RA) by comparing their relationship to that of Musa (RA) and Harun (RA), but making the obvious distinction Ali (RA) is will not succeed him as a Prophet. Rather he uses the word Caliph to signify his rank. The Prophet (SAW) doesn't mean Caliph
just in the political sense, but also in the spiritual sense. Ali (RA) is without doubt one of the leading inspirational figures in Islamic history. This form of leadership is more weighty than being charged with political leadership.
Likewise the Prophet (SAW) said about Umar (RA) to the nearest meaning "if there were to be a Prophet after me it would've been Umar". Similarly, the Prophet (SAW) is revealing the virtue of Umar. The Shias take the meaning of certain words in narrations too
rigid-literally, and totally shadow the context.
For the record the narration is not weak. The Hadith in has been recorded in the following primary Hadith collections:
1) Sunan Tirmidhi (Hadith: 3686)
2) Musnad Ahmad (Vol. 4 pg. 154)
3) Mustadrak Hakim (Vol. 3 pg. 85)
This Hadith is not weak.
Imam Tirmidhi (RA) has graded the Hadith as sound (hasan). Imam Hakim and Hafiz Dhahabi (RA) have declared the Hadith as authentic.
4) There are narrations where the Prophet (SAW) did not just hint Abu Bakr (RA) will be his first successor, but also prophesied his rein as he did with Umar (RA) and Uthman (RA). This idiot is sitting their confidence with his book collection like he put forward a 100 different narrations proving their point, lol. No doubt he ignored the original 6 classical Hadith literature for obvious reasons.
5) Finally, and strangely enough, I accept the narration in it's meaning that Ali (RA) is indeed the Caliph of the every believer after the Prophet (SAW) in every sense because he eventually did assume the Caliph, but it doesn't mean he is nor was the
only Caliph of the believers. In a nutshell the Khulafa Rashidun are the Caliph of all the believers. This is where out aqeedah differs with the Ithna Ashari Shia cult.