If you are creating a reusable tool, you do really not want to specify it in terms of high level operations that depend on a single library and may change from one version to another.
Just because those steps are explicit there, it does not mean the author isn't using crypto_box or something similar. The author seems to have just copied it into the code, and seems to have a good sense of what to copy.
You can criticize him for not reusing the standard interface, and I agree that if he wanted to use crypto_box, he should just have used it, but not for completely specifying his tool, and not for the actual operations.
Just because those steps are explicit there, it does not mean the author isn't using crypto_box or something similar. The author seems to have just copied it into the code, and seems to have a good sense of what to copy.
You can criticize him for not reusing the standard interface, and I agree that if he wanted to use crypto_box, he should just have used it, but not for completely specifying his tool, and not for the actual operations.