Martin’s idea about Codecs is to safely build decoder/encoder from small composable blocks. If you build a Codec only using his lib, you’ll have the GUARANTEE the decoder/encoder are in sync (except maybe only for customTypes I think, where the definitions are separate ; so you only have to be very careful for those kind of codec)!
It is a bit stronger than keeping encoder/decoder “close enough” (you have to be very careful at each codec definition).