Thanks @Ariel Glikman, great article, and samples on the OE app/GitHub repo.
I believe anyone deploying IAM in a production-grade environment must consider and would greatly benefit from, using one of these options, and this would be an excellent jump board.
Probably best you ask whoever sent you these classes.
In general though %JSON.Adaptor has the %JSONFIELDNAME parameter, and not JSONNAME. Hence the error you got.
[There is an existing package shared in the past on the Community, see for example here, with some similar functionality, that did have this JSONNAME parameter. But I don't know if this is related in any way to the code you have]
[Note I also see you are using a %DynamicObject property ("Rationale"), and depending on what you expect to do with it, I don't know how well that will play with the %JSON.Adaptor export, per your use-case.]
Wow! RESTFORMS was one of my all-time favorites!
Great contributions all around!
Well deserved recognition!