I also think INFPs mistype as INTJ more often than one might think.
As for 'noticeably confident' some are probably to the point of arrogance or even being pompous. I would also think it's situational as INTJs can become 'experts' in an area (self-appointed or not). Similar to how an INTP that is normally quiet and shy can suddenly become feisty and even trollish about a particular area of expertise.
But, you're just as likely to get a very unassuming INTJ. That doesn't mean a lack of confidence, just a lack of showiness.