Your quote appears to be from an article Graeme Barber wrote where he complained about the alterations WotC made to an adventure he wrote, so the idea of "good yuan-ti" is entirely his own and was ultimately not included in the final product, but you might be referring to his comment about the Tomb of Annihilation. As I am actually DM-ing this campaign currently, I am in a good position to answer your question. I will try to stay light on spoilers, but anyone who'd rather not have Tomb of Annihilation spoiled for them at all should not read the spoilered part.
First of all, no yuan-ti in Tomb of Annihilation are presented as
benevolent, and in fact all of them are presented as immoral and evil,
even if not all of them have to be fought. His comment about a schism
is partially correct, though. The yuan-ti presented are all part of a
single faction that holds a hidden stronghold in the jungle and
worships an apocalyptic deity. As part of their adventure, player
characters have the opportunity to ally themselves with a yuan-ti
character who is opposed to the leader of the faction, but that
character's opposition is primarily a matter of ambition. The two
characters have the same end goal, summoning their deity and bringing
about an apocalyptic scenario. The player characters don't have to
exterminate the yuan-ti because they are not relevant to their goal,
which is stopping the Death Curse, and the yuan-ti can be convinced
not to interfere with that goal, but that does not mean the yuan-ti
are benevolent or even allies.
This is consistent with their general depiction in 5e: "Humanoid
emotions are foreign to most yuan-ti, which understand sentiment only
as an exploitable weakness." Now, that description does say "most", so
it is possible that some yuan-ti aren't like this, but even if they
feel emotions, empathy might not be included among those. It is worth
pointing out that player characters in the Tomb of Annihilation have
the opportunity to transform into yuan-ti purebloods themselves, and
the only change to their psyche listed is that they gain a random
indefinite madness, so it is possible that neutral-aligned or even
heroic yuan-ti might exist, but every single major yuan-ti society in
the Forgotten Realms that has been presented so far seems to be
actively and monstrously evil, and frankly this makes sense for a
civilization whose progenitors deliberately transformed themselves
through foul rituals that involve human(oid) sacrifice.
The very fact Graeme's idea was edited out of existence for the final product might imply that the very idea of a good yuan-ti society has no place in the Forgotten Realms in the minds of the WotC writers. Personally, I would agree with this assessment, for the most part. Consider the example of the drow, who are not innately evil, but who have such a dominant and cruel society (dictated by a very active goddess) that examples of good drow are rare. Now, the yuan-ti gods are typically far less active, but they do have innate psychological differences over most humanoids, and an equally (if not more so) dominant and cruel society. I would conclude that good or even neutral yuan-ti would be very rare indeed, and when accounting for the overall low population of yuan-ti in the Forgotten Realms, finding even a small society of non-evil yuan-ti would be almost miraculous. Now, the reason I keep mentioning the Forgotten Realms is because yuan-ti in other settings could have entirely different psyches and cultures. I think I saw Keith Baker (the creator of Eberron) talking about how the yuan-ti of Eberron could be influenced by Couatls and would thus have a predisposition for good alignments rather than evil ones.
