Obviously what you said is not some unheard-of secret or deep analysis. It is a running joke inside the Google system.
But... many Googlers who have a tendency of repeating these things have (1) not seen how shittier things are on the outside (2) are not in management and do not know how much manager lies to them (3) have unrealistic expectations of how well any process applied to 100-200k people can work. If you see a place that has a better overall promo system, you'll almost certainly find that it is a much smaller shop and things are decided more ad-hoc at the top with higher information flow.
Specifically, for (2) the manager and their adjacent group can clearly flag the slipping under the rug behavior and ding one's promo. However, sometimes when they message it back they would lie about it to the employee and blame some other management or requirement or complexity, etc. Other times, the manager is a "people manager" moron and non-technical, and can't really evaluate (in which case it's not the process that's at fault, but useless management.)
It's also not clear that the optimal quality is achieved by spending more time "perfecting" things. At Google, people already work much less than other companies. Perhaps the answer is in fact the opposite: pushing to ship more milestones per unit of time and driving harder to then perfect it. My bet is if the promo packet was accepted without a full "launch"[1] they would have still shipped the same half-baked crap at a later point in time.
[1]: many years ago, they wanted to reduced half-baked "launches" and said we want "landings" not "launches" and wrote some documents explaining the difference and self-congratulated themselves. Net result: s/launch/landing in promo packets.