The point is that writing software that uses cryptography is unlike writing software that, say, uses a Twitter API library. In the latter case you can detect errors quite easily because functionality will be compromised if you make a mistake. This is not so in programs that use cryptography; they tend to work without noticeable problems, but they may be shooting themselves in the foot.

It's all about preventing known mistakes. For example, say a developer uses a One Time Pad encryption scheme in their application and calls it "secure". Surely if we've had any contact with cryptography we know that a OTP with a reused pad is broken and leaks key bytes, and vulnerable to many other types of attack.

These mistakes can be avoided by having proper education in cryptography. I understand cryptocat's developers urge to write it in the same way they've written software the rest of their lives, through trial and error. And that is okay for most software, but not cryptography.