No, it is not illegal under 7 USC 136j (a)(2)(G) to use a pesticide labeled for use against one pest against another pest... at least not anymore.
It turns out that there apparently were cases of the EPA interpreting the language of that portion of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) in an overly broad manner. This included the interpretation in question here, as well as even interpreting it to ban using doses or concentrations less than those specified by the label.
In response, Congress amended FIFRA in 1978 to add a definition of the term "To use any registered pesticide in a manner inconsistent with its labeling" as 7 USC 136 (ee):
The term “to use any registered pesticide in a manner inconsistent with its labeling” means to use any registered pesticide in a manner not permitted by the labeling, except that the term shall not include
(1) applying a pesticide at any dosage, concentration, or frequency less than that specified on the labeling unless the labeling specifically prohibits deviation from the specified dosage, concentration, or frequency,
(2) applying a pesticide against any target pest not specified on the labeling if the application is to the crop, animal, or site specified on the labeling, unless the Administrator has required that the labeling specifically state that the pesticide may be used only for the pests specified on the labeling after the Administrator has determined that the use of the pesticide against other pests would cause an unreasonable adverse effect on the environment...
[other exceptions not relevant to this question]
The act that included the amendment became Public Law 95-396 (PDF), also known as the Federal Pesticide Act of 1978, upon being signed into law by President Carter on Sept 30, 1978.
A question was raised on the linked question that this exception might have been referring to accidental use against one pest when another was being targeted, however, the following description of the meaning was given in a summary of the bill on the Senate floor when the final version of the bill was being considered for passage:
Third, the new definition will permit farmers to use pesticides that are already registered for a crop or site for pests not listed on the labeling. It is rather foolish to tell farmers that they can put a pesticide on a crop for one bug, but that they cannot apply it to the same crop for another bug.
Senator Herman Talmadge (D-GA), Senate Floor, Sept 18, 1978
(Source: Congressional Record vol. 124 (1978), part 22, page 29760 (Warning: 584 MB PDF))
Senator Talmadge was Chair of the Senate Committee on Agriculture, Nutrition, and Forestry at the time and so had been heavily involved in crafting the bill.
So, allowing the use of a registered pesticide against a pest not mentioned on its label was indeed the expressed intent of Congress in adding this exception to the definition of using a pesticide in a manner inconsistent with its labeling.
As long as the pesticide in question is being applied to a plant, animal, or site specified on the label and the Administrator of the EPA hasn't required the specific pesticide in question to expressly prohibit use against other pests on its label, then intentionally using it against a pest not specified on its label is not a violation of 7 USC 136j (a)(2)(G).
