Got to remember the Thalmor also claimed to have ended the Oblivion crisis. Kind of odd to take the word on things from simple mistakes such as those. Including their view on Talos with those few appearances of him.
The other Gods are mad because Sithis would dominate them. So they don't let the Bully in their club
Honestly Sithis has a right to be considered a god due to what he is refereed to.
The Nine are considered to be connected to certain things, in a same way Sithis is connected to things of the same nature.
In other words, it would not make sense to see him as a con of the Nine to keep things balanced.