Where TypedCommand is introduced in commands.h, we have a description of what is required of a Request type.

https://github.com/10gen/mongo/blob/master/src/mongo/db/commands.h#L1324-L1349

Due to changes in command requirements this description is now incomplete.
It is missing several new and subtle requirements.

The changes that need to be included are, at least:

Must now have member functions:
setGenericArguments(GenericArguments),
getGenericArguments()
getGenericArguments() const

There is the addition of a DeserializationContext* in the parse static member as well.

The IDL generates "command" classes that meet these requirements, but IDL is not a specification. It's an implementation of the specification. The specification has become incomplete.