Maybe they meant in 0g?

You need “downwards” acceleration for a swing. The vacuum bit isn’t important, it’s the gravity. They probably meant 0g rather than vacuum.

none, but centers of gravity do

It slows you down :)

The vacuum part was an emphasis on requiring a counterpart to change momentum.

In an atmosphere you could flap your wings, or use electric fans, and argue what it means to ride a swing, but it would be besides the point, as in an atmosphere there is no need for propellant-less engines from the article.