Where did it say that 刹那现象生灭观 is not true? When you think of 无常, how frequent should this 无常 happen? If 无常 is not an instant event, should it happen in a second, a minute, a day, a year, a million years, or what? You can see 无常 in an instant, as in meditative experiences, with 刹那现象 生灭.