It is always interesting to me when people say they are "bad test takers". You mean you are bad at the part where we find out how much you know? Maybe you just don't know the material well enough.
caveat emptor - I am not ND so maybe this is a real concern for some, but in my experience the people who said this did not know the material. And the accommodations for tests are abused by rich kids more than they are utilized by those that need them.
As a self proclaimed bad test taker, it's not that I don't know the information. It's that I am capable of second guessing myself in a particular way in which I can build a logical framework to suggest another direction or answer.
This presents itself as a bad test taker, I rarely ever got above a B+ on any difficult test material. But you put me in a lab, and that same skillset becomes a major advantage.
Minds come in a variety of configurations, id suggest considering that before taking your own experience as the definitive.
datum: I'm ND, but I'm a good test-taker. There were plenty of tests for subjects where I didn't need to study because I was adept at reading the question and correctly assuming what the test-creator wanted answered, and using deduction to reduce possibilities down enough that I could be certain of an answer - or by using meta-knowledge of where the material from the recent lectures was to narrow things down, again, not because I knew the material all that well but because I could read the question. Effectively, I had a decent grasp of the "game" of test-taking, which is rather orthogonal to the actual knowledge of the class material.
I think the reverse exists as well. I think I am a much better test taker than average, and this has very clearly given me some advantages that come from the structure of exam-focused education. Exam taking is a skill and it's possible to be good at it, independent of the underlying knowledge. Of course knowing the material is still required.
However you are correct in noticing that there are an anomalously high number of "bad test takers" in the world. Many students are probably using this as a flimsy excuse for poor performance. Overall I think the phenomenon does exist.
Tests are just a proxy for understanding and/or application of a concept. Being good at the proxy doesn’t necessarily mean you understand the concept, just like not being good at the proxy doesn’t mean you don’t. Finding other proxies we can use allows for decoupling knowledge from a specific proxy metric.
If I was evaluating the health of various companies, I wouldn’t use one metric for all of them, as company health is kind of an abstract concept and any specific metric would not give me a very good overall picture and there are multiple ways for a company to be healthy/successful. Same with people.
There are lots of different ways to utilize knowledge in real world scenarios, so someone could be bad at testing and bad at some types of related jobs but good at other types of related jobs. So unless “test taking” as a skill is what is being evaluated, it isn’t necessary to be the primary evaluation tool.