The CTK is anticipating the 'engagedParty' node to be an object, but according to the TMF629 documentation, a single customer can have multiple 'engagedParty' resources, as reflected in the provided examples. I have included screenshots of the examples from the TMF629 documentation and the cURL response from the CTK in case the 'engagedParty' is represented as an array. It seems there is a discrepancy between the expected data structure by the CTK and the documented specifications in TMF629.
Version used - v4.0.0
It's a defect in the example. A Customer (a specialization of PartyRole) must by definition have exactly one engagedParty - which is the party (individual or organization) that is playing the role (of customer in this case, or indeed any other role).
The examples were fixed in the v5 API assets, as you can see in the Open API table https://www.tmforum.org/oda/open-apis/table
In general, the examples are illustrative not normative - its the swagger/OAS file that actually defines the model. We do try to validate the examples against the schema, and in v5 this has been done much more rigorously.
Hope it helps