If you use a solid stopper it will seal and create a vacuum within the carboy... if you use vodka or starsan in an airlock, eventually it will suck it all through you will still be sucking air through once it runs out of vodka/starsan to suck through ... not sure why he shouldn't follow my advice, this is what I've been doing for more than 8 years and haven't had an issue.
I've only been doing it since '91 or so. Neither the S lock nor the 3-piece will suck back through if they are filled correctly. If they are not they will suck some through (which is why I recommend vodka) and then stop.
I have experimented with using solid stoppers in empty carboys (which creates a larger vacuum because of the volume of air) and with new stoppers I can get them to seal ~75% of the time and of those they suck in about 50% of the time. With older stoppers where I assume the rubber is hardened, nearly 90% of them fail and pass air. I share this because most people are not obsessive-compulsive enough to try all this themselves.
Now *if* you can get a good seal, and *if* you keep from pulling the stopper in, then it's a superior method IMHO because it will also de-gas the beer at the same time. This is not often done by brewers but our wine-making brethren know it will definitely help a liquid clear faster. For this I have used the orange rubber caps, the plugs that come with them, and a hose clamp around the top. The more headspace is in the carboy the greater the effect. If there is liquid to the top you will likely not develop any vacuum.
So, apologies for what sounded like an extremely dismissive treatment of your advice. It was pre-coffee and not meant to be offensive. I'm just going by my experience. I feel for most people that simply leaving a perfectly good airlock on there, avoiding the opportunity to infect the beer by switching stoppers, and using something which will not have any negative effects if suckled in, will be the better solution.
Incidentally, I tried this with a 6 gallon Better Bottle and their stopper/airlock setup. 5 gallons of product in a 6 gallon BB will leave enough head space to create a profound vacuum that will start to collapse the bottle.
ETA (since you edited after I began to reply):
I'm not the only one with this suggestion:
Simply because you can find other people doing the same thing does not necessarily mean it is correct or better.
A refrigerator is a horribly dirty place, full of all sorts of spoilage microorganisms. My method assumes a large possibility of not forming a good seal. That being the case I do not want to create a channel with which gravity and in-rushing air will be utilized by said nasties to enter my beer. An airlock is not a great method of sanitizing air, but it's better than nothing. It also, as I said above, avoids a step which could also introduce bugs. The less touch points the better IMHO, but there's more than one way to skin a cat.
Fact is, a lot of us over-engineer our solutions to the point that benefits are lost to the effort required to gain them. We made perfectly fine beer back when we didn't know to cold crash.