But then just a few lines later, it tells us:
While not really part of the Worms of Taut "family," they can be summoned by those who know of the Worms and Pit Vipers, although that knowledge is very rare.
The denial about their counting as a worm of Taut is somewhat confusing - if they are not, why would they "distant cousins to the Tri-Fangs" in the first place - but i tend to interpret the text as meaning to say that, yes, a Circle of Summon Serpents, that is uniquely suited to summon the worms, may also be used to call Pit Vipers, by those familiar with them (not many, as the creature is somewhat obscure outside of Hades) and the Worms and this particular way of summoning them.
But i would like to hear other people's opinions, exactly because i'm all too aware of how one's milleage may vary in that kind of thing.
PS: As an aside, do you think the Circle of Summon Serpents or some particular tweak or twist on it might be used to summon serpentine D-Bees, like the Naga from Pantheons of the Megaverse, the Pythonans from Splynn Dimensional Market, Vernulians from D-bees of North America and others?