Our own Jock Scot once shared an anecdote about wearing the bottom button open and being sternly corrected at a High Street shop in Inverness so your idea that it isn't necessarily the Highland convention is not without support or merit. In practice, however, I think it's a coin toss. Many famous for regularly wearing Highland attire leave it open and many leave it closed. Some like Lochiel and Rothsay seem to do either as the mood or outfit suits them.

MacRobert's approach makes some sense. On a three button waistcoat, it would seem a waste to leave one open and hide the third silver button. On a flat bottom waistcoat, there is no advantage to opening it, but on a longer, pointed waistcoat either to facilitate sitting, or for other practical reasons, I see leaving it open as a viable option.

So, with that in mind, I'm not so sure, Kinloch Anderson got it wrong per se. I think they made a deliberate fashion choice. Remember, they have been leading Highland fashion trends for a century, not necessarily just following them. They designed many of the kilt jackets we see today (coatee, Kenmore/Balmoral doublet), although admittedly their creations were an evolution from previous designs.