I've seen people get frustrated by the "don't"s before, but I think that's generally taking the first degree approximation too literally. Feel free to hand-write assembly or roll your own crypto, but don't depend on it for anything serious unless you are an expert or have it reviewed by one. Doing so for learning and fun is fine, if that's clearly called out such that no one accidentally depends on it for something serious. There's only one way to become good at something, and that's good practice!

In a professional setting there's a responsibility to the end user which generally precludes doing these dangerous things - that is, one should feel free to take up woodworking as a hobby but shouldn't offer to build someone's house unless you're a licensed professional.