That's not really the point of the saying though. The point is more so that if you are the kind of person who is in a position to roll your own crypto you probably have a PhD and have worked with cryptography for years. And the thing that you're setting out to do is to produce a cryptography library.
If you're that kind of person, then sure go for it! But if you're a top person in the field of cryptography you didn't need anyone to tell you if you were fit to make a cryptography library anyway.
And if you're not that kind of person you do need someone to tell you because the whole field is a minefield where subtle oversights break the whole thing. If you want to just implement cryptography as a fun learning experience that's great. But actual implementations that people will actually depend on need to be handled by experts.
