I think the point you're missing is that senior engs in Meta do not just write code. Some of the senior engs almost do not write code at all. They design systems and move complex projects forward. So that's more of like a TPM/PM/mgmt skill there, in addition to excellent technical ability to write the code itself. In certain projects, especially in infra, writing code is usually the last part (and often the least complex one) of your avg large project driven by senior ICs; and often the code is written by engs across teams/orgs, not necessarily the senior eng themselves.
So yea, deep language knowledge, while an excellent skill, is not something that defines a senior eng (at least in Meta and I presume in similar FAANG companies).