Afaik to be correct you always need quotes around identifiers, if the parse works also without we could call this a loophole? 
According to the BNF:

````
<id predicate> ::= [ "NOT" ] "IN" "(" <id> {"," <id> } ")"
<id> ::=  <character string literal>
````

A character string equal is always between quotes, otherwise it can be takes as a variable name