...I think the speculation that domain authority doesn't exists is far from the truth.
It's not speculation. It's fact.
Gary Illyes confirmed there is no such thing as domain authority,
[seroundtable.com]. I posted an article on my site with the results of my research and study on whether a thing like domain authority might exist in a Google algo. My research concluded that no such thing as domain authority is in use at Google.
You'll find the article if you search on Google: martinibuster domain authority
The concept of Domain Authority started in the early days of SEO. In the beginning, domains did have authority and would be able to rank for a large number of phrases because of their PageRank. Google noticed that and changed their algorithm to rely less on PageRank. That was a long time ago. Yet, the concept remained, even though Google's algorithm had changed.
This is a good example of a Rote SEO belief. SEOs believe something because it's attained a canonical level of belief, like a holy book, and everyone's afraid to question it. (Canonical means a list of holy books that are accepted to be beyond doubt as the real writings.) So everyone keeps believing in domain authority even though Google changed the algo around 2003 and the change was plain to see.
I did the research and shared it. We even have a statement from a Googler who has said it's not true. We have the history of the SERPs to show where the belief came from and how the circumstances changed.
Continued belief in Domain Authority is... strange and not logical, imo.