They're talking about sum types (aka tagged unions aka variant record), where you have a proper type with a closed set of variants (which can be checked for completeness by the compiler) and optional associated data (possibly of different types for each variant).
Iota isn't even a step up from C's enums, it doesn't come close to what you can find in ML descendants (such as Rust or Swift for the most recent ones).
They're talking about sum types (aka tagged unions aka variant record), where you have a proper type with a closed set of variants (which can be checked for completeness by the compiler) and optional associated data (possibly of different types for each variant).
Iota isn't even a step up from C's enums, it doesn't come close to what you can find in ML descendants (such as Rust or Swift for the most recent ones).