Well, you need both. You are going to suck at implementing algorithms if you don't learn how to test they actually work as intended.

Most algorithms are used in library form. Unless you are writing those libraries, you probably should not be reinventing a lot of wheels. So there's a valid argument there that most of the stuff you learn as part of your computer science courses, you will not be implementing over and over again (if ever). So, you could argue that testing is a more universal skill that you need either way.

But algorithms can come up once in a while. And it helps if you can guesstimate complexity of various algorithms and make some trade offs over picking one or the other. The skill you learn in college is not any particular algorithm but a broad knowledge of which mainstream ones are there, how they work, their tradeoffs, and the skill of implementing those or similar algorithms.

You gain the skill of good judgment, being able to figure out how stuff works, and general intuition of how things are done at a high level. I've never developed a file system. But I know tree data structures such as b trees and red black trees have something to do with it. It's been decades since I looked at that stuff. But I could read up in an afternoon or so if it comes up. That doesn't qualify to start working on a file system. But, I don't think that's going to come up anyway. I have plenty of other things to do.

I do dabble a bit with search algorithms once in a while. More of a hobby than work related. But there's some room in that space for being able to do some basic things with algorithms instead of using some prefab search product.