How do I pass a list of strings as a parameter in CloudFormation?

As @MaiKaY points out, the flaw in @Liam Mayfair's code is that Fn::Split is preceded by - which results in a list containing a single element which is a list. The fixed code would look like

...
            Resource:
              Fn::Split:
                - ","
                - !Ref S3Buckets

On a more general note, you must make sure to use a Parameter type of String not CommaDelimitedList when using Fn::Split as it won't split a CommaDelimitedList.

• If you use CommaDelimitedList with Fn::Split you'll get the error Template error: every Fn::Split object requires two parameters, (1) a string delimiter and (2) a string to be split or a function that returns a string to be split
• If you use CommaDelimitedList with no Fn::Split you'll get the error Syntax errors in policy

Because the return value of !Split is A list of string values. I would do it in the following way:

[...]
    Resource: !Split [",", !Ref S3Buckets]
[...]