Earlier quoted context omitted.
Poems aren't transient. Oral tradition is.
But Twitter isn't "oral tradition" by any definition of the word. I dare you to find any property that can be said of written poems with respect to those being transcendent, which can't also be said of Twitter threads (other than "they've existed since the beginning of writing").
A poem written on Twitter and deleted the next day is no more permanent or long-lasting than a poem that's solely available in the oral tradition, perhaps much less so.
Why are epics such as Beowulf or the Epic of Gilgamesh considered literature while others aren't? It's because someone at some point recorded them in a more permanent manner so future generations could read them in their unadulterated form (as much as that can be said of stories that probably had been changed and embellished multiple times already before someone wrote them down).
A thread on Twitter however can be deleted, added to, remixed, retweeted and forked at any time, which certainly are very interesting properties for creating art but I'm not sure if that art should be called literature or rather something else entirely.
Anyway, as originally stated what does and doesn't constitute literature isn't very precisely defined. So, this probably is hairsplitting anyway.