Promotion decisions are made by committees which are 1-2 levels above your manager, your manager presents the candidates. They round up a pot of multiple teams which are discussed at once and there are usually hard quotas (like 5%) of promotions to give out to this pot of employees. These hard quotas make it impossible to "do the right thing" because even if a lot of people deserve the promotion, only x% can get it. The composition of the pot of people can easily cause the problem which is described in the blog post, for example if you have a high number of juniors or a high number of employees who joined at the same time or employees with incorrect levelling from the start. If 20%+ deserve a promotion then it simply turns into a game of luck.
As a manager you try as hard as possible to get these promotions but the system of these big companies is just too rigid. Its like a pit fight instead of objectively looking at output. I have seen a lot of people leave for the same reason but I haven't seen a single change to the system in 5+ years.
Next we could talk about layoff mechanics, its equally disturbing.