"Never use self-published sources as third-party sources about any living people, except for claims by the author about themself. This holds even if the author of the source is an expert, well-known professional researcher, or writer."
That limits things quite severely. The relevant policy is here:
https://en.wikipedia.org/wiki/Wikipedia:Biographies_of_livin...
(Just to explain: in Wikipedia, "policies" are "widely accepted standards that all editors should normally follow"; a "guideline" is "a generally accepted set of best practices that editors should follow, though it is best treated with common sense, and occasional exceptions may apply", and an "essay" can be just one editor's opinion; it is "not one of Wikipedia's policies or guidelines, as it has not been thoroughly vetted by the community.")
I still disagree in the narrow bounds of this conversation, which is articles about technical topics. The prohibition there seems to be on accepting someone's claims about themselves, which is different from an expert on a subject making specific fact-based claims in their field.
> The author is an established expert on the topic of the article whose work in the relevant field has previously been published by reliable third-party publications, except for exceptional claims.[4] Take care when using such sources: if the information in question is really worth reporting, someone else will probably have done so.[5]