I would definitely tend towards this explanation. I too, would agree, that with places that do one-shot pours tend to pour bad Guinnesses. I also think these sorts of places probably serve a lot less Guinness, and therefore it isn’t as fresh, because surely the Guinness aficionados will flock to places that follow the ritual. Just pure speculation on my part. Like I said, within a single bar, having a Guinness pulled all-at-once or in two parts, I haven’t discerned any difference in taste.