How far should we take that maxim? It implies that no one should ever attempt this, but that leads to nothing new (unless you are first recognised as a crypto guru -- but how could you become one?).
I think it's worth drawing a distinction between the algos/maths and attempts at implementations. Otherwise we wouldn't have things like OCaml-TLS and others.
Implementations are even more sensitive to tiny bugs with huge consequences than algorithms. It's fine for people to write their own implementations if they're never used, but anything that will be used needs a large number of experts and a large amount of time before it should be trusted.
I think it's worth drawing a distinction between the algos/maths and attempts at implementations. Otherwise we wouldn't have things like OCaml-TLS and others.
http://openmirage.org/blog/introducing-ocaml-tls