I didn’t look through the study, but that high level bullet summary on the first page doesn’t make it clear if respondents were answering about internal or external APIs? For internal APIs, I think the standards of documentation are much lower for many companies. I think many people also feel like for internal APIs, they’re well designed/simple enough that they’re self evident, or that as developers are consumers, folks can refer to the code if necessary. I think we see similar with internal test frameworks - folks generally don’t document it, but then start wondering why there’s not good adoption/the automated tests don’t use the features of the framework/etc.
My current company has customer facing APIs, and we’re only now (year 7) getting around to worrying about dev experience. Previously, we relied heavily on auto-generated Swagger docs and lots of hand-holding during the tech-integration process, as well as a lot of institutional knowledge and random Word documents, sample requests, etc that got shared. Definitely a mess, and we’re trying to get more mature about this, relying less upon individuals in tech integration and trying to rely more upon well written documentation that tries to address workflows and other shortcomings that Swagger based documentation doesn’t provide.