Where is there an FAA source that states that squall lines are non-frontal?
If you go back and look at the previous version of that AC, AC-006A, the index has this definition:

squall line - Any nonfrontal line or narrow band of active
thunderstorms (with or without squalls).

I guess that the question in the question bank was based on AC-006A, which was current until 2016. As for today, chapter 12 of the PHAK has a slightly different definition (p.12-23). It says that squall lines are not necessarily frontal:

Squall Line
A squall line is a narrow band of active thunderstorms. Often it develops on or ahead of a cold front in moist, unstable air, but it may develop in unstable air far removed from any front.

If you need more detailed information on squall lines and fronts you may get a better response on earthscience.SE.