Haskell and, IIRC, Rust allow you to declare that a certain data type conforms to some interface, and describe how, by listing / adding the functions with necessary signatures.
This allows to have the upsides of structural polymorphism without losing static checks.
I think they're just talking about how you have to declare what trait a function implementation is for, rather than having it derived from the type signature alone. The `impl Trait`[0] syntax. In Go, you don't need to declare that the function implementations are being implemented for a particular interface, you just have to match the type signatures and function names.
Rust's way can help avoid some errors. You can't accidentally implement an interface, whereas in Go you can if you happen to implement a group of functions with appropriate names and type signatures. It's unlikely to cause actual bugs (you'd have to misuse the resulting implementation) but can be conceptually somewhat confusing.
This allows to have the upsides of structural polymorphism without losing static checks.
Go, OTOH, goes all the way structural.