"Never roll your own cryptography", despite the ambiguity of its phrasing, means never use uncertified crypto in production unless you're willing to take the risk. It doesn't mean to never build cryptography without an expert's permission. Everyone who has ever published world-class cryptography has rolled their own crypto.

What that phrase means is you shouldn't use hobby crypto libraries for any important production services. But that's also not the point behind "Build Your Own" type projects. So your advice isn't correct in this scenario.