Specifically, he claims: that large portions of the Swift user interface API are entirely undocumented; that he was unable understand the full capabilities and limitations of the package manager through reference to its documentation; that he finds himself frequently searching video transcripts to find information unavailable in the documentation.
I think that, if these claims are true, it does not reflect a good state of affairs, and that the extra work imposed on the individual developer is worth complaining about.
I don't think that he is calling out the work of the technical writers in any unsociable or shameful fashion in the text of his blog post. I agree that 'garbage' is not a good metaphor for "woefully inadequate," as it implies that the existing documentation would be better disposed of than referred to. However, I think that when dealing with a large software corporation that has teams dedicated to writing and maintaining technical documentation, it is not especially cruel or unfair to use hyperbolic language in calling extraordinary lapses to public attention.
The actual, in-production APIs are documented.