Every line of code written can be a severe vulnerability, not just code dealing with crypto. In case of crypto, being well-informed is good enough to not mess up those kind of implementations (on a theoretical level). The problems mentioned can be pointed out and verified on paper. But you can't say the same thing for code.
No, you can write crypto code, even when knowing stuff about the crypto, and still mess up. Crypto is problematic because you often cannot easily check whether you're wrong (it could be a tiny edge case that isn't tested because your input space is huge).
I didn't deny messing up crypto code is a thing. I'm saying messing up code is a separate problem from messing up how one architects projects using crypto.
What you say as messing up with edge cases, etc. is a programming problem. The errors talked about in the link he posted is about using faulty crypto. Today it is common knowledge to never use ECB.
What cipher mode you use is a problem you think about before you even start writing a single line of code. But you are talking about programming errors, which one can make in all areas, not only ones involving crypto. Those errors can break security similar to messing up crypto implementation.
