I think you can use whatever rules you want for your fictional truth serum. Of course in his right mind, Matt would be horrified, but if you want the serum could make him relaxed and unaware of what he's doing.

Though I'm wondering if you mean this prompt: http://daredevilkink.dreamwidth.org/1742.html?thread=3705550#cmt3705550

Because that prompt specifically mentions Matt being "smiley and chatty," so I think the way you're writing it (with Matt being chill about it) is what OP wanted.