That’s true, at least at the outset of a foam-making excursion. An egg foam won’t form well if there’s much fat present in the mixture. However you can bring plenty of fat to the party after the foam is formed with few ill effects. Why?
It all has to do with proteins, specifically egg proteins, the things that make a long-lasting egg white foams possible. These molecules occur bunched up in an egg white, but are uncoiled (“denatured” as they say in the hip, beatbox parlance of food chemistry) by whipping them. These long and languid molecules, it’s important to note, aren’t uniform. They have different structures along their length, some of which are attract to air (hydrophobic or “water hating”) and others that are attracted to water (hydrophilic or “water loving”). The surface of an air bubble is therefore an ideal spot for them. They can stick their water loving parts in water, their air loving parts in air, and because they have other parts that like to bond to each other, they can hang onto other protein molecules in the bargain. Taken together, it all adds up to a bubble-encapsulating mesh that reduces surface tension and keeps the bubble from popping.
Fats have similar water- and air-loving parts, and if allowed into the mixture early, will compete for spots on the bubble surfaces, interrupting protein networks and rendering the bubble more likely to burst. However fat molecules can’t break up protein networks once they’re formed, so you can add plenty to, say, a chocolate mousse or a soufflé batter without wrecking the foam. You can, however, weigh it down with enough fat, which is what happens in a buttercream. A good deal of the bubbles pop due to their sheer volume and weight of all that butter. Enough remain, however, to keep a meringue buttercream quite light in texture compared to its non-meringue relatives.