Why are input and output of I2C voltage level shifters connected?
It looks like an error.

When the supply voltages are close enough together it may not be necessary to level shift I2C, just put the pull-up resistors to the lower supply rail.

It is required that the lower supply rail meets the Vih requirement of the devices fed with the higher supply rail.

Since both sets of devices pull down to ground there is no issue in the low state.

Interfacing 5V and 3.3v devices is marginal.  Usually the data sheet specifies the worst case input high for I2C devices as 0.7 * Vcc, this is 3.5V - above the 3.3V supply rail that the pull-ups would be connected to. Typically it would work as the threshold would be nearer 0.5*Vcc which is below 3.3V.