When you're the guy trying to convince a room full of people to change their religion, . . .
Trying to ascertain a person's motivations is always a risky venture, but I'd be dumbfounded if Parker (who has been around the block here a few times) harbors that hope still at this point. There are those naive souls who are led to believe that if one but lays out the "clear truth of Scripture" before us Scripturally illiterate, blinded Catholics that we'll stumble to answer and the scales will fall from our eyes. Those types typically will venture confidently into the discussion and retreat immediately upon realizing how mistaken that assumption was.
I think at Parker's stage the person is more out to show (to him/herself or friends) that Catholicism rests upon a shaky foundation and can't be Bibically supported. It's more a matter of self-justification than conversion. I think that's why many never really venture outside the "sola scripture" box they inhabit. They aren't really trying to challenge the foundations of Catholic belief; they are merely trying to "prove" that Catholics can't defend their beliefs according to the standards the non-Catholic critics have set up, viz., the Bible only, as intepreted by that person or those who think like that person. So the Catholic can even cite to Protestant Bible commentaries and the present critic will typically just dismiss it as meaningless.
Parker here was actually trying to take on Catholics on a topic we consider a strong suit (history/Tradition). I don't think he proved in the least what he attempted. Probably he thinks otherwise.
the burden of proof is on YOU...
We each have our varying approaches to these discussions. Mine is a simple one: 1) answer the question asked (if possible) and then 2) ask the person even harder questions. At this point, Parker is laying low. He says he's busy with work. These discussions can indeed be time-consuming (though when it's a thread you've started, my view is there's a bit of implied obligation to stick with it). Though Catholic critics seem to "get busy" right about the time they move from the offensive to the defensive posture on a topic. Call me a cynic.