This post is a bit old, but I wanted to add a few things since I came across it.
Keep in mind that there are plot holes in The Elder Scrolls lore (and Bethesda could make mistakes).
1. It seems more likely that they are serpents since it is said that they ate all of the "men" on Akavir. Plus its unfair to choose either the book vs other evidence and ignore the other.
2. The "men" could have been enslaved to fight in their armies (consume being a metaphor) or the Tsaesci could shape shift.
3. ESO describes them as serpents (and even shows a picture of a helmet as well as one item with a description describing them having scaled throats).
5. When the Tsaesci are shown it could instead be referring to the ancient Akaviri (Blades). When does Alduin's Wall ever say anything (I don't recall Esbern ever saying anything, though I could be mistaken)?
Do we really need another human race, we can have a serpent one? (Like the Tzik'nith)
There's no definitive answer yet, but I still stand by the book Mysterious Akavir.
There are other sources and videos on youtube that support this theory, although too many to list.