Buck Controller ICs: Does the rated input voltage range limit the buck circuit supply voltage?
There are switcher ICs that expect you to provide the switches externally.  This is sometimes for the reason you state, which is to allow for a higher input voltage than the IC can handle.  Sometimes it is to allow for higher current than can reasonably pass thru a IC.  Unfortunately for you, the IC you show is intended for the latter case.

Note that the H output of the IC in your schematic actually needs to go higher than the 400 V input.  It is driving the gate of a N channel FET.  Your theory of the IC not seeing the high voltage is therefore incorrect.  It should also be obvious that the SW pin will see the high voltage.

You need to actually read the datasheet.  Right on page 1 is shows a typical schematic.  Even though the switches are external to the IC, clearly the IC has to handle the input voltage on several different pins.  Note the charge pump to make the extra high voltage for the driving the top FET.