I felt like I could have used a better articulation of their definition of "loud" and "quiet" aliens, unless these are well known terms in the domain terms and I am just unfamiliar...
The abstracts of papers are intentionally terse, which usually means they are addressed to people already familiar with the field. The paper itself is often more readable (though by no means always!) and it is often worth taking a look at the introduction and conclusion, at least. Here, we have 'To a first approximation, there are two kinds of aliens: quiet and loud. Loud (or “expansive”) aliens expand fast, last long, and make visible changes to their volumes. Quiet
aliens fail to meet at least one of these criteria.'