So, I think I quite got it. When Input is 1 Volts output is 5 volts. (as the datasheet says).
When input is 1.1 Volts the output is 5 - 25*0.1 = 2.5 Volts (as you said), and when input is 1.2 volts the output is 0 volts. Beyond this the amplifier has already saturated so the output remains 0 volts.
So, the state switching occurs at about 1.2 volts when increasing voltage from 0 to 5.
But, when I decrease voltage from 5 to 0, the stage switching occurs at about 3.8 Volts (by following similar analysis).
If these are true, then isn't the input behaving like Schmitt input?