But if the carrier chooses to throttle an "unlimited" data plan and thus degrades the delivery speed outside of "network management" contexts (such as congested network conditions) then they are now subject to additional punitive fines arising from net neutrality regulations that they are now subject to, on top of any (successful) fines related to misrepresentative advertising.

This falls under the category of willfully "impairing, or degrading" an existing service motivated by business interests related to the specific subscriber, instead of driven by purely technical network management justifications which would apply to a wider geographic area (i.e. not subscriber-specific), and this is one of the bright-line examples given by the FCC when they issued their press release.

As a side note, I believe the Federal Trade Commission (FTC) Bureau of Consumer Protection handles deceptive advertising enforcement, while the FCC handles net neutrality enforcement.