That's exactly the case for me. I spend physical cash with much less hesitation than for online purchases. I'd probably spend way more freely online if I didn't have to keep track of the purchase just in case something goes wrong, like getting mis-charged, double-billed, a continuing subscription I wasn't expecting, or just to remember why that $1 debit appears on my statement.
As an aside:
It's curious to see that this made it to the HN front page and already has more than 25 comments, yet nobody has commented on the original article's own page which does allow commenting. Well, there is one comment that says, "Great analysis", but that was me just to check if commenting actually works. I suppose that ties in with the same idea -- it's not worth the "cognitive cost" to make a comment no one will read.
Their comment form requires login or signup. I'm already signed up and logged in here. That's reason enough for me to post here and not there. I would never create an account just to leave a comment unless it was someplace where I intended to visit and comment often.
Though PayPal doesn't feel like that, I guess being in the browser is off-putting in terms of spending because of added risk.