Hi Teh,
Sorry for I have never checked sample code before.
However, according to my exploration from the sample code just now, I can conclude that there is no special Marketing SDK library,
actually "Microsoft.Dynamics.Marketing.SDK" is just a custom namespace by sample file provider(Microsoft).
For the first sample in doc:
https://docs.microsoft.com/en-us/dynamics365/marketing/developer/extend-customer-journey-using-code#examples
The CheckForErrors function will finally execute an organization request(validationRequest.OrganizationRequest),
the validationRequest variable is passed from FromCustomerJourneyEntity method of ValidationRequest class.
When we open ValidationRequest.cs source file, we can find the fact from its constructor method:
Actually the core part is "msdyncrm_CustomerJourneyValidate" request,

this request has been mentioned in article Work with Customer Journey API,
the overall operation is orgService.execute(orgRequest) indeed,
there is no special dll file for marketing entities,
what we need to do is to know some requests which are dedicated from Marketing application.
So my suggestion is that you could start with Web API version, then convert it into organizationRequest in C# version,
create a simple console app with organization request by yourself.
Regards,
Clofly