The author clearly does not understand that the Boland Amendment was budgetary boilerplate. It had the force of law as a condition upon the expenditure of appropriations. It had no standing as far as criminal law or anything else. If violated, the remedy would have been for a lawsuit to be filed to require that the money in question (funding for the White House) be spent properly.
Cheney's argument was that the condition could not apply to money that was not appropriated by Congress. That's not a power grab, it's a logical conclusion from the fact that it was a condition on money budgeted by Congress, and Iran-Contra involved money NOT budgeted by Congress.
The other argument by the Democrats at the time was that the language barred Contra-support activities by paid NSC staff, but even that's an iffy argument, as what constitutes paid activities as opposed to unpaid activities?