The more nuanced message is maybe don't write crypto code without having spent the right amount of time to understand the challenges. If you don't know about timing attacks, you aren't familiar with the many historical mistakes that are well known, you don't know any theory around ciphers or hashes, then you probably shouldn't be writing crypto code that is used to secure anything important. There are a lot of ways to get up to speed (Coursera's cryptography courses from Stanford, books etc.) to at least wrap your head around why this is a problem. Even the experts go through peer review cycles because they know how easy it is to make mistakes.

In other news, don't print yourself a motorcycle helmet on your 3d printer, don't make your own car tires, don't make your own break pads. I mean if you really know what you're doing maybe.

EDIT: I have some personal experience with screwing up some crypto code. I won't get into all the details but I was overconfident. And this was after taking graduate level cryptography courses and having spent considerable time keeping up to date. I didn't write anything from scratch but I used existing algorithms/implementations in a problematic way that left some vulnerability. This was a type of system where just using something off the shelf was not an option.