There are lots of things that it helps to know to be a good tester. The specifics will vary by job. And even if the specifics of this job are not directly applicable to the next job, the fact that you were interested and able to learn implies that you are valuable as a tester.
There is another kind of knowing that will also help you as a tester: knowing that your assumptions are wrong. We spend our careers building up a repertoire of assumptions that allow us to make quick decisions, because life without assumptions is difficult, e.g. the sun rises every morning, it's safe to cross the road when the traffic light is green, Joe's code probably works, and if Bob's code doesn't work, it's probably a problem in his database code. As a test person, you must be willing to question your assumptions over and over again. This skill it is intangible, hard to detect in an interview, and orthogonal to your technical skills. Nonetheless, it is a hallmark of a great tester.