I think there's a point of view component.
If you think of it is "Today I learned how to use $PROPRIETARY_AMAZON_TOOL to set up a new project which will use $PAT to deploy to a $PROPRIETARY_AMAZON_INTERNAL_CLOUD and sets everything up to $PA_MONITORING and $PA_ALERTING in my... etc. etc. etc., you get the idea, then yes, you will feel like what you "learned" is not transferrable.
On the other hand, if you look at it like "Today I learned some best practices for setting up a new industrial-strength projects through the particular manifestation of this tool, learned about setting up monitoring through the particular manifestation of this other tool,", etc. etc., then even though you may have learned via particular local tools, you also learned a lot of stuff that may be useful later.
The question is, let's say you did change jobs and had to learn some new set of tools. Will you learn them faster than you learned the first set of tools? Will you find a new perspective on the issues from learning the new set of tools? The answer should generally be yes. This is broadly speaking why I expect a 5-year veteran to pick things up faster than a fresh grad; OK, sure, you've never "used git", but I expect you've used source control somewhere. You may not "know React", but I expect you to have used something similar enough that you just have to learn React qua React and are not also learning about cookies and the basics of the DOM, etc.
Pretty much the first professional thing I ever did was writing what we would today call a "static site generator" in a tool called Frontier, in its custom programming language, more or less from scratch. On the one hand, completely useless skill, Frontier has been dead for over a decade and was fairly zombie-ish for a good 5-8 years before that. On the other hand... the HTML basics I learned are still mostly good (I think it's fair to say at this point my frameset tricks & skills are not that useful anymore...), and the general skill of knowing how to write a static site generator is almost even low-level trendy right now, even though I would have to write it in some other language, of course.
I dunno what the exact transfer number is. 90% seems high, certainly, but 10% way too low. There's only a small number of tools that are so bad that while working with them, you learn almost nothing about the problem space you are working in because the quirks of the tool completely dominate, and even then I'd submit a lot of it is still attitude.