the thing is, if you follow his advice, that you would shoot 1200iso in daylight and 400 at night.
so why constructors spend time and money building dual iso cameras ? as most of us need high iso to shoot at night and low to shoot in daylight.
people doing photography are used to zone system, at least the one that shot print film.
you just need to set the exposure for the result you want more than for standards exposures.
with film we used to expose as wanted and we processed the films with order in order to complete the process