In the half-bridge topology, the voltage going to the transformer is half the input voltage
the input of the transformer is switched between full input voltage and Gnd, that is correct, but the other end of the transformer is connected to a capacitive divider, C2 and C3.
There you have 1/2 of the input voltage (if C2and C3 have the same value), so the transformer only sees half the input voltage, either V+ to V+/2 when M2 is on (polarity = + on top of transformer - on bottom) or V+/2 to ground if M1 is on (polarity = - on top of transformer + on bottom)