You can learn everything most devs need to know about Kafka in a couple hours. I can't imagine why companies would ever hire based on current knowledge rather than problem solving skills and ability to learn.
We might be unique, but here's no situation we're using Kafka where someone unfamiliar with it would be able to design and support production level systems to the standards we do after a couple hours. That would result in a time bomb for someone else to clean up later when all the stuff you weren't aware you didn't know becomes a problem. These skills are common, some would say table stakes, so it's not hard finding them as long as you look outside the proprietary tech shops like FAANG, which is why I have no reason to prefer people that don't already have them.
My initial reaction reading this is that you really screwed up somewhere with your infrastructure. A pretty large company needs a Kafka team, a smaller company needs their infrastructure team to have a decent understanding with maybe one person / consultant with deeper knowledge.
Kafka for 99% of developers in the company should be "Can I use our internal library to read/write some topic".
My preferred size of company would not have the resources for a team dedicated to infrastructure. In companies this size, there would be multiple developers skilled enough with the tech to support it fully from installation, configuration, optimization, backup, DR, upgrades, troubleshooting, designing and implementing solutions, etc. Everything. I've worked at several companies of this size and much prefer the broader scope of responsibility and experience to what you get somewhere like you described.
We don't train people on or write our own documentation for any widely used open source tools. We use them as they're provided and don't expect any unique knowledge beyond that. That's a large part of the value in choosing tech like this instead of proprietary solutions we'd have to document and train people on. It's also why a developer with those skills can take them with them from job to job over the course of their career and they're still valuable or start at a new company and make valuable contributions from day one before they learn the proprietary bits of the company.