When I spoke generically of a 'list' I was referring to
all attribute that supports multiple values. Two of the attribute Data type in Tinderbox support multiple values:
List or
Set types. I commend following those links and reading the articles and exploring out from there in aTbRef. The main difference between Lists and Sets is sets do not support duplicate values. If the 2 value of a List or Set were "Cat;Dog" (semi-colon is the value delimiter) then adding the value "Cat" would result in a List value of "Cat;Dog;Cat" but a Set would have the result "Cat;Dog" as the duplicate is ignored.
$Authors needs to be multi-value as a reference (book, paper, etc.) often has—outside fiction—more than one author. More specifically it is a Set-type attribute as duplicates in the attributes stored value(s) would be inappropriate.
So I don't see a mistake her. If you look up what Set and List type do you'll see they are appropriate and that Set is a correct data type with $Authors.
The real issue here is that a single item list is still, under the hood, see a list consisting of one item. When using list attributes (i.e. List or Set data types) in action code you need to be aware that a reference to the attribute, e.g.$Authors, reference all values and you may need to use
.format() to turn the value(s) into a string