Why 100W max because this IC designed for 100W operation, Internal Gate Driver Bias Voltage of IC is about 12 V this voltage cant drive Power MOSFET, with use of a gate driver it is possible.
For Flyback application below 100W is reasonable because flyback topology required MOSFET current ratting 5.5times required current. For example 1000W offline flybac(200V input), input current will be 5A so we need to select 27.5A MOSFET at 400V or above, this mosfet will be cost high. So above 100W application other forward topologies are selecting.