Which version of the FAQ are you looking at? I think the answer above was superseded. V1.1 (current version on GW website) has the following updated (magenta) entry.
Q: How do you determine what models and units make up a warscroll battalion? When is it based on a keyword and when is it a unit name?
A: When a warscroll battalion is referring to a keyword, it appears in Keyword Bold. Otherwise, it is referring to the name of a unit.
When you read the composition of the Verminus Clawpack it states 1 Skaven Warlord (not in bold) and is therefore referring to the unit/warscroll. This prevents the use of Tretch and other named Warlords being used as part of the battalion.