Nate, this has been a very confusing matter for quite some time. Maybe you might take a look at this:
Reformed Apologist: A Formal Blunder on Van Til by the WTJ, no less!
It addesses the matter. The bottom line, TAG is a kind of deductive argument. It need not take the form of Modus Tollens; nor can it take the form that Collett suggests, which I interact with.
I think the Bahnsen quote you referenced simply means that whether you argue for or against a transcendental, the transcendental must be affirmed being a transcendental. So, to deny logic or to affirm logic, logic must be employed.
Ron