This part i agree with.
This part i dont, exactly. the human and orc have no idea who this pandaren is (or probably even WHAT he is at that moment) so it would seem more likely that the pandarens intentions in fighting them are unclear. (ie: they dont know hes trying to stop them, just that hes fighting, and fighting well)
for me the last part would go more like:
Human: uhh i dont know whats happening but this thing is kicking BOTH our asses. we should probably deal with this first.
Orc: Agreed.
i dont see it as any sort of grand gesture of "alliance and horde setting aside their differences and working together" like some people seem to.
but there is a sort of implied rule when you are trying to fight someone, which is that you dont just want to win, but also that you dont want yourself to get killed in the process.
the pandaren had already shown he could take them both on. so it was either "deal with this thing first then get back to what we were doing", or "im so blind with hatred that ill just let the panda kill us both because WAR WAR WAR". the later is just awful two dimensional writing. but oddly seems to be what most people that dislike the cinematic seem to want.
You really can't expect any form of logic from anything Blizzard puts out.
Just roll with it.
"Do not only practice your art, but force yourself into its secrets, for it and knowledge can raise men to the divine." -- Ludwig Van Beethoven
I was in a similar situation when I visited Australia:
I was boxing a kangaroo when a croc came up and almost got us, We turned and attacked the croc because it was perceived to be the more immediate threat. It actually turned out to be a guy in a mascot costume trying to get me out of the kangaroo enclosure but at the time we both thought it was trying to kill us and so we both savagely attacked the guy to near death.