One of the nice things about open source, and modern development tools, is the ability to easily go beyond the documentation to explore the library code itself. While I wouldn't consider that as a substitute for implementing your own, it is another means of figuring out how a library works and can be seen as more results oriented. Yet it is nowhere near as interesting, in my opinion.
As an early career developer, your team wants you to write useful code with few errors and that works best when you can rely on established libraries.
But as your career progresses, your value comes from understanding how those libraries work and from being capable of writing new libraries that support and stabilize the early career developers coming up behind you.
And the only way to go from the first group to the second is from getting your hands dirty. Writing personal and hobby projects that “reinvent the wheel” is one of the best ways to do that.