everyone wants the tsaesci to be beastly snake-men, and are in a fit with the prospect that the in-game books may very well be fictional and untrue even in-game. afterall, the dwemer were suggested as actual dwarves in arena and daggerfall, and even the ayleid culture was suggested as completely different.
i bet that the tsaesci are men, probably asian-esque. their slanted eyes and tanned, yellow skin is alien to the people of tamriel, their armor, probably splintmail looked serpentine, scaly. calling them snake-men could no doubt just be old fashioned tamriel racism.
It waits to be seen, at the current point, but if that actually happens, I will start to be angered at Bethesda for butchering lore, I may even take the stance on "butchered lore" some fans seem to use of disregarding the new lore they do not like in favor of the old lore, making Dwemer into a race of techno-elves whose name was misinterpreted as "Dwarves" helped to enrich the lore and make it more appealing to me, but turning the Akaviri snake men into another generic race of Not-Asians with some vague associations with snakes would just serve to make the Elder Scrolls world a more bland place, in the mean time, they will always be immortal vampire snakes to me, anything less would leave me sourly disappointed with Bethesda.
As to whether they are Akaviri, it really doesn't matter what they are, genetically, they live in Akavir, and have remained there for as long as we know, by all accounts, they are people of that continent, and are therefore Akaviri, after all, we don't say that real people living in specific parts of the world are not actually people from that place because their ancestors migrated from somewhere else long ago, I wouldn't, at least.