Is this a remake of "just spend 15 days near a whiteboard, then write the code and it will be awesome" ?

Have you ever heard of the V model (https://en.wikipedia.org/wiki/V-model) ?

Long story short: it does not work, because the act of coding adds new valuable inputs that alters the (original) design.

One might say "you just have to know", which is an equivalent to "you just have to figure out the future", "you just have to be perfect" and other non-sense (which does explain why V cycle does not work, high inefficient, low quality outcome, bad method)