This is tricky, because we already have some airside people movers in the browser.
I'm open to whatever, but personally, I count airside people movers as a transit system in my own stats. They are short, sure, but they exist and aren't that much shorter than some of the lines in the browser already. Heck, the ATL Plane Train is longer than multiple routes we already count. I also don't love counting landside people movers and not airside ones; this becomes splitting hairs solely based on access instead of what it is. Anyone can buy a plane ticket and get on the train. This isn't like the US Capitol Subway or other private/highly restricted systems, which you need to be a member or employee of Congress to ride.
My standard of significance would be "can anyone ride this without being an employee or guest somewhere?" If the only barrier is purchasing a ticket, it's no harder than riding some long-distance rail lines, which we are already including in some countries.